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# Q-Chem

Q-Chem is a comprehensive ab initio quantum chemistry package for accurate predictions of molecular structures, reactivities, and vibrational, electronic and NMR spectra.

## Setup

You first need to install a working copy of Q-Chem for ASE to call; follow the instructions on the Q-Chem website.

The default command that ASE will use to start Q-Chem is `qchem PREFIX.inp PREFIX.out`.

## Examples

An simple example of running a geometry optimization using the QChem calculator in the python interface:

    from ase.build import molecule
    from ase.calculators.qchem import QChem
    from ase.optimize import LBFGS

    mol = molecule('C2H6')
    calc = QChem(label='calc/ethane',
                 method='B3LYP',
                 basis='6-31+G*')
    opt = LBFGS(mol)
    opt.run()

See the source code link below for further details.

class ase.calculators.qchem.QChem(restart=None, ignore_bad_restart_file=<object object>, label='qchem', scratch=None, np=1, nt=1, pbs=False, basisfile=None, ecpfile=None, atoms=None, **kwargs)

  QChem calculator

  The scratch directory, number of processor and threads as well as a few other command line options can be set using the arguments explained below. The remaining kwargs are copied as options to the input file. The calculator will convert these options to upper case (Q-Chem standard) when writing the input file.

  scratch: str

    path of the scratch directory

  np: int

    number of processors for the -np command line flag

  nt: int

    number of threads for the -nt command line flag

  pbs: boolean

    command line flag for pbs scheduler (see Q-Chem manual)

  basisfile: str

    path to file containing the basis. Use in combination with basis=’gen’ keyword argument.

  ecpfile: str

    path to file containing the effective core potential. Use in combination with ecp=’gen’ keyword argument.