# Molecule and Geometry Specification

## Coordinates

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 optional_molecule_name {...}` block.

    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.

## Molecule Keywords

In addition to specifying the geometry, additional information can be provided in the molecule block `molecule optional_molecule_name {...}`.

Charge & Multiplicity
  If two integers `charge multiplicity` are encountered on any line of the molecule block, they are interpreted as the molecular charge and multiplicity (), respectively. For multi-fragment complexes, each fragment can have a `charge multiplicity` line.
Units
  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`.
Orientation
  Certain computations require that the molecule is not reoriented. This can be achieved by adding either `no_reorient` or `noreorient`. To prevent even recentering of the molecule, add `no_com` or `nocom`.
PubChem
  A line reading `pubchem:mol` fetches the geometry for molecule `mol` from the PubChem database, where `mol` is either the IUPAC molecule name or the CID number. See PubChem Database for details.
Symmetry
  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, one of one of `c1`, `c2`, `ci`, `cs`, `d2`, `c2h`, `c2v`, or `d2h`. This need not be specified, as the molecular symmetry is automatically detected by Psi4. See Symmetry for details.
Fragments
  A line reading `--` is interpreted as the separator between two non-covalently bound molecular fragments. See Non-Covalently Bonded Molecule Fragments for details.

## 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')

    clean()

    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 command is provided. With it, 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')

    clean()

    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. Job Control Keywords provides more details about the job control and calculation keywords used in the above examples.

## Ghost Atoms

While many common computations, particularly 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.

## 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 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 three doubly occupied orbitals, as well as one 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 molecule keyword `symmetry symbol` can be used (see Molecule Keywords). In most cases the standard Schönflies symbol (one of `c1`, `c2`, `ci`, `cs`, `d2`, `c2h`, `c2v`, `d2h` will suffice for `symbol`. 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')

If the molecule contains fragments but is not conveniently ordered for the `--` marker, the auto_fragment function can be applied, as shoown in pywrap-basis, to return as active molecule the previous active molecule, only fragmented.

## Advanced Python

A named molecule in an input file is a full-fledged instance of the powerful C++ Molecule class. Thus, all member functions (that have been exported via Boost Python) documented thereat are accessible through the handle `option_molecule_name` in `molecule optional_molecule_name {...}`.