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# Psithon: Structuring an Input File

To allow arbitrarily complex computations to be performed, PSI4 was built upon the Python interpreter. However, to make the input syntax simpler, some pre-processing of the input file is performed before it is interpreted, resulting in Python syntax that is customized for PSI, termed Psithon. In this section we will describe the essential features of the Psithon language. PSI4 is distributed with an extensive test suite, described in section Test Suite and Sample Inputs; the input files for these test cases can be found in the samples subdirectory of the top-level PSI4 source directory, and should serve as useful examples.

## Physical Constants

For convenience, the Python interpreter will execute the contents of the `~/.psi4rc` file in the current user’s home area (if present) before performing any tasks in the input file. This allows frequently used python variables to be automatically defined in all input files. For example, if we repeatedly make use of the universal gravitational constant, the following line could be placed in the `~/.psi4rc` file

    UGC = 6.67384E-11 # m^3 / kg^-1 s^-2

which would make the variable `UGC` available in all PSI4 input files. For convenience, the physical constants used within the PSI4 code (which are obtained from the 3rd edition of the IUPAC Green book [Cohen:GreenBook:2008]) are also automatically loaded as Psithon variables (before `~/.psi4rc` is loaded, so that `~/.psi4rc` values can be overridden by the user).

The physical constants used within PSI4, which are automatically made available within all PSI4 input files.

    psi_h                        = 6.62606896E-34       # The Planck constant (Js)
    psi_c                        = 2.99792458E8         # Speed of light (ms$^{-1}$)
    psi_kb                       = 1.3806504E-23        # The Boltzmann constant (JK$^{-1}$)
    psi_R                        = 8.314472             # Universal gas constant (JK$^{-1}$mol$^{-1}$)
    psi_bohr2angstroms           = 0.52917720859        # Bohr to Angstroms conversion factor
    psi_bohr2m                   = 0.52917720859E-10    # Bohr to meters conversion factor
    psi_bohr2cm                  = 0.52917720859E-8     # Bohr to centimeters conversion factor
    psi_amu2g                    = 1.660538782E-24      # Atomic mass units to grams conversion factor
    psi_amu2kg                   = 1.660538782E-27      # Atomic mass units to kg conversion factor
    psi_au2amu                   = 5.485799097E-4       # Atomic units (m$@@e$) to atomic mass units conversion factor
    psi_hartree2J                = 4.359744E-18         # Hartree to joule conversion factor
    psi_hartree2aJ               = 4.359744             # Hartree to attojoule (10$^{-18}$J) conversion factor
    psi_cal2J                    = 4.184                # Calorie to joule conversion factor
    psi_dipmom_au2si             = 8.47835281E-30       # Atomic units to SI units (Cm) conversion factor for dipoles
    psi_dipmom_au2debye          = 2.54174623           # Atomic units to Debye conversion factor for dipoles
    psi_dipmom_debye2si          = 3.335640952E-30      # Debye to SI units (Cm) conversion factor for dipoles
    psi_c_au                     = 137.035999679        # Speed of light in atomic units
    psi_hartree2ev               = 27.21138             # Hartree to eV conversion factor
    psi_hartree2wavenumbers      = 219474.6             # Hartree to cm$^{-1}$ conversion factor
    psi_hartree2kcalmol          = 627.5095             # Hartree to kcal mol$^{-1}$ conversion factor
    psi_hartree2MHz              = 6.579684E9           # Hartree to MHz conversion factor
    psi_kcalmol2wavenumbers      = 349.7551             # kcal mol$^{-1}$ to cm$^{-1}$ conversion factor
    psi_e0                       = 8.854187817E-12      # Vacuum permittivity (Fm$^{-1}$)
    psi_na                       = 6.02214179E23        # Avagadro's number
    psi_me                       = 9.10938215E-31       # Electron rest mass (in kg)

The `psi_` prefix is to prevent clashes with user-defined variables in PSI4 input files.

## Molecule and Geometry Specification

PSI4 has a very flexible input parser that allows the user to provide geometries as Cartesian coordinates, Z-matrix variables, or a combination of both. The use of fixed values and variables are supported for both. For example, the geometry for H2 can be specified a number of ways, using the `molecule` keyword:

    molecule{
      H
      H 1 0.9
    }

     or

    molecule{
      H
      H 1 r
      r = 0.9
    }

     or

    molecule{
      H1
      H2 H1 0.9
    }

     or

    molecule{
      H 0.0 0.0 0.0
      H 0.0 0.0 0.9
    }

     or

    molecule{
      H 0.0 0.0 0.0
      H 0.0 0.0 r
      r = 0.9
    }

     or

    molecule{
      H 0.0 0.0 -r
      H 0.0 0.0 r
      r = 0.45
    }

Blank lines are ignored and, unlike regular Python syntax, indentation within the `molecule` block does not matter, although the `molecule` keyword itself must be aligned within the input according to standard Python syntax. For more examples of geometry specification, see the mints1 input file in the samples folder. It is also possible to mix Cartesian and Z-matrix geometry specifications, as demonstrated in the mints4 and mints6 sample input files. For example, consider the following geometry specification, taken from the mints6 input:

    molecule alanine {
        N           -1.527107413251     0.745960643462     0.766603000356
        C           -0.075844098953     0.811790225041     0.711418672248
        C            0.503195220163    -0.247849447550    -0.215671574613
        O           -0.351261319421    -0.748978309671    -1.089590304723
        O            1.639498336738    -0.571249748886    -0.174705953194
        H           -1.207655674855    -0.365913941094    -0.918035522052
        # First, remove the H from the alpha carbon.  This line could be deleted
        # and is only included for completeness
        #H            0.429560656538     0.717651915252     1.673774709694
        # Now patch it, using a Z Matrix specification.  This patch can be applied
        # anywhere in the coord specification, as long as it appears lower than
        # the atoms referenced, as is usual for Z-Matrices
        C  2  rCC   3  aCCC   1  dCCCN
        H  7  rCH1  2  aHCC1  3  dHCCC1
        H  7  rCH2  2  aHCC2  3  dHCCC2
        H  7  rCH3  2  aHCC3  3  dHCCC3
        H            0.221781602033     1.772570540211     0.286988509018
        H           -1.833601608592     0.108401996052     1.481873213172
        H           -1.925572581453     1.640882152784     0.986471814808

        aCCC = 108.0
        rCC = 1.4
        dCCCN = 120
        rCH1 = 1.08
        rCH2 = 1.08
        rCH3 = 1.08
        aHCC1 = 109.0
        aHCC2 = 109.0
        aHCC3 = 109.0
        dHCCC1 = 0.0
        dHCCC2 = 120.0
        dHCCC3 = 240.0
    }

Here, we remove the hydrogen from the alpha carbon of glycine and replace it with a methyl group. Applying this patch using Cartesian coordinates is difficult, because it depends on the orientation of the existing glycine unit. In this example, we use Z-Matrix coordinates to define the methyl group, and define the orientation in terms of the existing glycine Cartesian coordinates which is much easier to visualize than the corresponding Cartesian-only approach.

### Multiple Molecules

To facilitate more elaborate computations, it is possible to provide a name for each molecule, and tell PSI4 which one should be used in a given calculation. For example, consider the following input file:

    molecule h2{
      H
      H 1 0.9
    }

    set basis cc-pvdz
    set reference rhf
    energy('scf')

    molecule h{
      H
    }

    set basis cc-pvdz
    set reference uhf
    energy('scf')

Here, two separate jobs are performed on two different molecules; the first is performed on H2, while the second is for H atom. The last molecule to be specified is the “active” molecule by default. To explicitly activate a named molecule, the activate keyword is provided. Using this keyword, the above input file can be equivalently written as follows:

    molecule h2{
      H
      H 1 0.9
    }

    molecule h{
      H
    }

    activate(h2)
    set basis cc-pvdz
    set reference rhf
    energy('scf')

    activate(h)
    set basis cc-pvdz
    set reference uhf
    energy('scf')

Note that whenever the molecule is changed, the basis set must be specified again. The following section provides more details about the job control keywords used in the above examples.

### Molecule Keywords

In addition to specifying the geometry, additional information can be provided in the `molecule optional_molecule_name {...}` block. If two integers `charge multiplicity` are encountered on any line of the molecule block, they are interpreted as the molecular charge and multiplicity (), respectively. The symmetry can be specified by a line reading `symmetry symbol`, where `symbol` is the Schönflies symbol of the (Abelian) point group to use for the computation. This need not be specified, as the molecular symmetry is automatically detected by PSI4. Certain computations require that the molecule is not reoriented; this can be achieved by adding either `no_reorient` or `noreorient`. By default, Ångström units are used; this is changed by adding a line that reads `units spec`, where `spec` is one of `ang`, `angstrom`, `a.u.`, `au`, or `bohr`.

### Ghost Atoms

While many common computations, such as SAPT and counterpoise corrections, can be greatly simplified using the notation described in Non-Covalently Bonded Molecule Fragments, manual specification of ghost atoms is sometimes required. Either

    molecule he2 {
        He
        Gh(He) 1 2.0
    }

or

    molecule he2 {
        He
        @He 1 2.0
    }

will generate a helium dimer, with the second atom ghosted, i.e., possessing basis functions but no electrons or nuclear charge. See dfmp2_1 and ghosts for a demonstration of both mechanisms for specifying ghost atoms.

## Geometries from the PubChem Database

Obtaining rough starting guess geometries can be burdensome. The Z-matrix coordinate system was designed to provide chemists with an intuitive method for guessing structures in terms of bond lengths and angles. While Z-matrix input is intuitive for small molecules with few degrees of freedom, it quickly becomes laborious as the system size grows. To obtain a reasonable starting guess geometry, PSI4 can take a chemical name as input; this is then used to attempt to retrieve Cartesian coordinates from the [PubChem] database.

For example, to run a computation on benzene, we can use the following molecule specification:

    molecule benzene {
        pubchem:benzene
    }

If the computer is connected to the internet, the above code will instruct PSI4 to search PubChem for a starting structure. The search is actually performed for compounds whose name contains “benzene”, so multiple entries will be returned. If the name provided (“benzene” in the above example) exactly matches one of the results, that entry will be used. If no exact match is found the results, along with a unique chemical identifier (CID), are printed to the output file, prompting the user to provide a more specific name. For example, if we know that we want to run a computation on a compound whose name(s) contain “benzene”, but we’re not sure of the exact IUPAC name, the following input can be used:

    molecule benzene {
        pubchem:benzene*
    }

Appending the “*” prevents an exact match from being found and, at the time of writing, the following results are displayed in the output file:

    Chemical ID     IUPAC Name
             241   benzene
            7371   benzenesulfonic acid
           91526   benzenesulfonate
             244   phenylmethanol
             727   1,2,3,4,5,6-hexachlorocyclohexane
             240   benzaldehyde
           65723   benzenesulfonohydrazide
           74296   N-phenylbenzenesulfonamide
             289   benzene-1,2-diol
             243   benzoic acid
            7370   benzenesulfonamide
          636822   1,2,4-trimethoxy-5-[(E)-prop-1-enyl]benzene
            7369   benzenesulfonyl chloride
           12932   N-[2-di(propan-2-yloxy)phosphinothioylsulfanylethyl]benzenesulfonamide
            7505   benzonitrile
           78438   N-[anilino(phenyl)phosphoryl]aniline
           12581   3-phenylpropanenitrile
          517327   sodium benzenesulfonate
          637563   1-methoxy-4-[(E)-prop-1-enyl]benzene
          252325   [(E)-prop-1-enyl]benzene

Note that some of these results do not contain the string “benzene”; these compounds have synonyms containing that text. We can now replace the “benzene*” in the input file with one of the above compounds using either the IUPAC name or the CID provided in the list, viz:

    molecule benzene {
        pubchem:637563
    }

     or

    molecule benzene {
        pubchem:1-methoxy-4-[(E)-prop-1-enyl]benzene
    }

Some of the structures in the database are quite loosely optimized and do not have the correct symmetry. Before starting the computation, PSI4 will check to see if the molecule is close to having each of the possible symmetries, and will adjust the structure accordingly so that the maximum symmetry is utilized.

The standard keywords, described in Sec. Molecule Keywords, can be used in conjuction to specify charge, multiplicity, symmetry to use, etc. .

## Symmetry

For efficiency, PSI4 can utilize the largest Abelian subgroup of the full point group of the molecule. Concomitantly a number of quantities, such as SOCC and DOCC, are arrays whose entries pertain to irreducible representations (irreps) of the molecular point group. Ordering of irreps follows the convention used in Cotton’s Chemical Applications of Group Theory, as detailed in Table Irreps. We refer to this convention as “Cotton Ordering” hereafter.

Ordering of irreducible representations (irreps) used in PSI4
Point Group | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8
--- | --- | --- | --- | --- | --- | --- | --- | ---

For example, water ( symmetry) has 3 doubly occupied orbitals, as well as 1 each of and symmetry; the corresponding DOCC array is therefore:

    DOCC = [3, 0, 1, 1]

Although PSI4 will detect the symmetry automatically, and use the largest possible Abelian subgroup, the user might want to run in a lower point group. To do this the `symmetry` keyword can be used when inputting the molecule (see Sec. Molecule and Geometry Specification). In most cases the standard Schönflies symbol (one of `c1`, `c2`, `ci`, `cs`, `d2`, `c2h`, `c2v`, `d2h` will suffice. For certain computations, the user might want to specify which particular subgroup is to be used by appending a unique axis specifier. For example when running a computation on a molecule with symmetry in , the axis can be chosen as either the , the , or the ; these can be specified by requesing the symmetry as `c2vx`, `c2vy`, or `c2vz`, respectively. Likewise the `c2x`, `c2y`, `c2z`, `c2hx`, `c2hy`, and `c2hz` labels are valid. For symmetry the labels `csx`, `csy`, and `csz` request the , , and planes be used as the mirror plane, respectively. If no unique axis is specified, PSI4 will choose an appropriate subgroup.

Certain types of finite difference computations, such as numerical vibrational frequencies, might lower the symmetry of the molecule. When this happens symmetry-dependent arrays, such as SOCC, are automatically remapped to the lower symmetry. For example, if we were to investigate the state of water cation, we can specify

> SOCC = [0, 0, 1, 0]

in the input file. If any ensuing computations lower the symmetry, the above array will be appropriately remapped. For example, reducing the symmetry to (with the molecular plane defining the mirror plane), the above array will be automatically interpreted as:

> SOCC = [0, 1]

Some caution is required, however. The state can be obtained with the

> SOCC = [1, 0, 0, 0]

specification, which would become

> SOCC = [1, 0]

under the above-mentioned reduction in symmetry. The state, whose singly-occupied orbitals are

> SOCC = [0, 0, 0, 1]

would be mapped to

> SOCC = [1, 0]

which is the same occupation as the state. In this case, the state is lower in energy, and is not problematic. The distorted geometries for the state are excited states that are subject to variational collapse. One way to obtain reliable energies for these states is to use a multi-state method; in this case it’s easier to run the entire computation in the lowest symmetry needed during the finite difference procedure.

## Non-Covalently Bonded Molecule Fragments

PSI4 has an extensive range of tools for treating non-covalent intermolecular forces, including counterpoise corrections and symmetry adapted perturbation theory methods. These require the definition of which fragments are interacting within the complex. PSI4 provides a very simple mechanism for doing so; simply define the complex’s geometry using the standard Cartesian, Z-matrix, or mixture thereof, specifications and then place two dashes between nonbonded fragements. For example, to study the interaction energy of ethane and ethyne molecules, we can use the following molecule block:

    molecule eneyne {
      0 1
      C  0.000000 -0.667578  -2.124659
      C  0.000000  0.667578  -2.124659
      H  0.923621 -1.232253  -2.126185
      H -0.923621 -1.232253  -2.126185
      H -0.923621  1.232253  -2.126185
      H  0.923621  1.232253  -2.126185
      --
      0 1
      C 0.000000 0.000000 2.900503
      C 0.000000 0.000000 1.693240
      H 0.000000 0.000000 0.627352
      H 0.000000 0.000000 3.963929
    }

In this case, the charge and multiplicity of each interacting fragment is explicitly specified. If the charge and multiplicity are specified for the first fragment, it is assumed to be the same for all fragments. When considering interacting fragments, the overall charge is simply the sum of all fragment charges, and any unpaired electrons are assumed to be coupled to yield the highest possible value.

Having defined a molecule containing fragments like `eneyne` above, it is a simple matter to perform calculations on only a subset of the fragments. For instance, the commands below run a scf first on the ethene fragment alone (`extract_subsets(1)` pulls out fragment 1 as Real atoms and discards remaining fragments) and next on the ethene fragment with the ethyne fragment ghosted (`extract_subsets(1,2)` pulls out fragment 1 as Real atoms and sets fragment 2 as Ghost atoms). For beyond bimolecular complexes, arrays can be used, e.g. `extract_subsets(2,[1,3])`:

    mA = eneyne.extract_subsets(1)
    energy('scf')

    clean()

    mAcp = eneyne.extract_subsets(1,2)
    energy('scf')

## Job Control

PSI4 comprises a number of modules, written in C++, that each perform specific tasks and are callable directly from the Python front end. Each module recognizes specific keywords in the input file, detailed in Appendix Keywords by Module, which control its function. The keywords can be made global, or scoped to apply to certain specific modules. The following examples demonstrate some of the ways that global keywords can be specified:

    # all equivalent

    set globals basis cc-pVDZ

    set basis cc-pVDZ

    set globals basis = cc-pVDZ

    set basis = cc-pVDZ

    set globals{
      basis cc-pVDZ
    }

    set {
      basis cc-pVDZ
    }

    set {
      basis = cc-pVDZ
    }

Note the lack of quotes around `cc-pVDZ`, even though it is a string. The Psithon preprocessor automatically wraps any string values in `set` commands in strings. The last three examples provide a more convenient way for specifying multiple keywords:

    set {
      basis = cc-pVDZ
      print = 1
      reference = rhf
    }

For arguments that require an array input, standard Python list syntax should be used, viz.:

    set {
      docc = [3, 0, 1, 1]
    }

List/matrix inputs may span multiple lines, as long as the opening `[` is on the same line as the name of the keyword.

Any of the above keyword specifications can be scoped to individual modules, by adding the name of the module after the `set` keyword. Omitting the module name, or using the name `global` or `globals` will result in the keyword being applied to all modules. For example, in the following input

    molecule{
      o
      h 1 roh
      h 1 roh 2 ahoh

      roh = 0.957
      ahoh = 104.5
    }

    set basis cc-pVDZ
    set ccenergy print 3
    set scf print 1
    energy('ccsd')

the basis set is set to cc-pVDZ throughout, the SCF code will have a print level of 1 and the ccenergy code, which performs coupled cluster computations, will use a print level of 3. In this example a full CCSD computation is performed by running the SCF code first, then the coupled cluster modules; the `energy()` Python helper function ensures that this is performed correctly. Note that the Python interpreter executes commands in the order they appear in the input file, so if the last four commands in the above example were to read

    set basis cc-pVDZ
    energy('ccsd')
    set ccenergy print 3
    set scf print 1

the commands that set the print level would be ineffective, as they would be processed after the CCSD computation completes.

## Assigning Basis Sets

While the above syntax will suffice for specifying basis sets in most cases, the user may need to assign basis sets to specific atoms. To achieve this, a `basis` block can be used. We use a snippet from the mints2 sample input file, which performs a benzene SCF computation, to demonstrate this feature.

    basis {
       assign DZ
       assign C 3-21G
       assign H1 sto-3g
       assign C1 sto-3g
    }

The first line in this block assigns the DZ basis set to all atoms. The next line then assigns 3-21G to all carbon atoms, leaving the hydrogens with the DZ basis set. On the third line, the hydrogen atoms which have been specifically labelled as `H1` are given the STO-3G basis set, leaving the unlabelled hydrogen atoms with the DZ basis set. Likewise, the fourth line assigns the STO-3G basis set to just the carbon atoms labelled `C1`. This bizzare example was constructed to demonstrate the syntax, but the flexibility of the basis set specification is advantageous, for example, when selectivily omitting diffuse functions to make computations more tractable.

In the above example the basis sets have been assigned asymmetrically, reducing the effective symmetry from to ; PSI4 will detect this automatically and run in the appropriate point group. The same syntax can be used to specify basis sets other than that used to define orbitals. For example,

    set df_basis_mp2 cc-pvdz-ri

     or

    basis {
       assign cc-pVDZ-RI df_basis_mp2
    }

are both equivalent ways to set the auxiliary basis set for density fitted MP2 computations. To assign the aug-cc-pVDZ-RI to carbon atoms, the following command is used:

    basis {
       assign C aug-cc-pVDZ-RI df_basis_mp2
    }

When most popular basis sets are being used, including Dunning and Pople-style, the SCF, DF-MP2, and SAPT codes will chose the appropriate auxiliary basis set automatically according to Auxiliary Basis Sets, unless instructed otherwise by setting the auxiliary basis set in the input. Finally, we note that the `basis {...}` block may also be used for defining basis sets, as detailed in Sec. User-Defined Basis Sets.

## Memory Specification

By default, PSI4 assumes that 256 Mb of memory are available. While this is enough for many computations, many of the algorithms will perform better if more is available. To specify memory, the `memory` keyword should be used. The following lines are all equivalent methods for specifying that 2 Gb of RAM is available to PSI4:

    # all equivalent

    memory 2 Gb

    memory 2000 Mb

    memory 2000000 Kb

One convenient way to override the PSI4 default memory is to place a memory command in the `~/.psi4rc` file (Sec. Scratch Files and the ~/.psi4rc File). For example, the following makes the default memory 2 Gb.

    set_memory(2000000000)

However, unless you’re assured of having only one job running on a node at a time (and all nodes on the filesystem with `~/.psi4rc` have similar memory capacities), it is advised to set memory in the input file on a per-calculation basis.

Note

For parallel jobs, the `memory` keyword represents the total memory available to the job, not the memory per thread.

## Return Values and PSI Variables

To harness the power of Python, PSI4 makes the most pertinent results of each computation are made available to the Python interpreter for post-processing. To demonstrate, we can embellish the previous example of H2 and H atom:

    molecule h2{
      H
      H 1 0.9
    }

    set basis cc-pvdz
    set reference rhf
    h2_energy = energy('scf')

    molecule h{
      H
    }

    set basis cc-pvdz
    set reference uhf
    h_energy = energy('scf')

    D_e = psi_hartree2kcalmol*(2*h_energy - h2_energy)
    print"De=%f"%D_e

The `energy()` function returns the final result of the computation, the requested total energy in Hartrees, which we assign to a Python variable. The two energies are then converted to a dissociation energy and printed to the output file using standard Python notation.

Generally, there are multiple quantities of interest. Appendix PSI Variables by Module lists PSI variables variables set by each module. These can be accessed through the `get_variable()` function. For example, after performing a density fitted MP2 computation, both the spin component scaled energy and the unscaled MP2 energy are made available:

    e_mp2=get_variable('MP2 TOTAL ENERGY')
    e_scs_mp2=get_variable('SCS-MP2 TOTAL ENERGY')

Each module and the Python driver set PSI variables over the course of a calculation. The values for all can be printed in the output file with the input file command `print_variables()`. Note that PSI variables accumulate over a PSI4 instance and are not cleared by `clean()`. So if you run in a single input file a STO-3G FCI followed by a aug-cc-pVQZ SCF followed by a `print_variables()` command, the last will include both `SCF TOTAL ENERGY` and `FCI TOTAL ENERGY`. Don’t get excited that you got a high-quality calculation cheaply.

## Loops

Python provides many control structures, which can be used within PSI4 input files. For example, to loop over three basis sets, the following code can be used:

    basis_sets=["cc-pVDZ","cc-pVTZ","cc-pVQZ"]
    for basis_set in basis_sets:
        set basis = $basis_set
        energy('scf')

The declaration of `basis_set` is completely standard Python, as is the next line, which iterates over the list. However, because the Psithon preprocessor wraps strings in quotes by default, we have to tell it that `basis_set` is a Python variable, not a string, by prefixing it with a dollar sign.

The geometry specification supports delayed initialization of variable, which permits potential energy scans. As an example, we can scan both the angle and bond length in water:

    molecule h2o{
      O
      H1 R
      H1 R2 A
    }

    Rvals=[0.9,1.0,1.1]
    Avals=range(102,106,2)

    set basis cc-pvdz
    set scf e_convergence=11
    for R in Rvals:
        h2o.R = R
        for A in Avals:
            h2o.A = A
            energy('scf')

The declarations of `Rvals` and `Avals` are both completely standard Python syntax. Having named our molecule `h2o` we can then set the values of `R` and `A` within the loops. Note that we do not need the dollar sign to access the Python variable in this example; that is required only when using Python variables with the `set` keyword.

## Tables of Results

The results of computations can be compactly tabulated with the `Table()` Psithon function. For example, in the following potential energy surface scan for water

    molecule h2o {
      O
      H 1 R
      H 1 R 2 A
    }

    Rvals=[0.9,1.0,1.1]
    Avals=range(100,102,2)

    table=Table(rows=["R","A"], cols=["E(SCF)","E(SCS)","E(DFMP2)"])

    set basis cc-pvdz

    for R in Rvals:
        h2o.R = R
        for A in Avals:
            h2o.A = A
            energy('df-mp2')
            escf = get_variable('SCF TOTAL ENERGY')
            edfmp2 = get_variable('DF-MP2 TOTAL ENERGY')
            escsmp2 = get_variable('SCS-DF-MP2 TOTAL ENERGY')
            table[R][A] = [escf, escsmp2, edfmp2]

    print table
    relative=table.copy()
    relative.absolute_to_relative()
    print relative

we first define a table (on line 10) with two row indices and three column indices. As the potential energy scan is performed, the results are stored (line 22) and the final table is printed to the output file (line 24). The table is converted from absolute energies to relative energies (in kcal mol-1) on line 26, before being printed again. The relative energies are reported with respect to the lowest value in each column. More examples of how to control the formatting of the tables can be found in the sample input files provided; see Appendix Test Suite and Sample Inputs for a complete listing.

## Python Wrappers

The Python foundations of the PSI4 driver and Psithon syntax permit many commonly performed post-processing procedures to be integrated into the PSI4 suite. Among these are automated computations of interaction energies through `cp()`, of a model chemistry applied to a database of systems through `database()`, and of several model chemistries together approximating greater accuracy through `complete_basis_set()`. These are discussed separately in section Psithon Functions: Invoking a Calculation. Note that the options documented for Python functions are placed as arguments in the command that calls the function, not in the `set globals` block or with any other `set` command.

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