psi4

psi4 is an open source quatum chemistry code out of the Sherill Group at Georgia Tech.

class ase.calculators.psi4.Psi4(restart=None, ignore_bad_restart=False, label='psi4-calc', atoms=None, command=None, **kwargs)[source]

An ase calculator for the popular open source Q-chem code psi4. method is the generic input for whatever method you wish to use, thus and quantum chemistry method implemented in psi4 can be input (i.e. ccsd(t))

also note that you can always use the in-built psi4 module through: calc.psi4

Setup

First we need to install psi4. There are instructions available on their website for compiling the best possible version of psi4. However, the easiest way to obtain psi4 by obtaining the binary package from conda:

conda install psi4 -c psi4; conda update psi4 -c psi4

The ase calculator operates using the psi4 python API, meaning that if psi4 is installed correctly you won’t need to do anything else to get psi4 working. It is, however, recommended that you set up a psi4 scratch directory by setting the PSI_SCRATCH environment variable:

export PSI_SCRATCH=/path/to/existing/writable/local-not-network/directory/for/scratch/files

This directory is where temporary electronic structure files will be written. It is important that this directory be located on the same machine as the calculation is being done to avoid slow read/write operations. This is set to /tmp by default. However, be aware that the /tmp directory might not be large enough.

Examples

You can import psi4 and run it like any other calculator in ase:

from ase.calculators.psi4 import Psi4
from ase.build import molecule
import numpy as np

atoms = molecule('H2O')

calc = Psi4(atoms = atoms,
        method = 'b3lyp',
        memory = '500MB' # this is the default, be aware!
        basis = '6-311g_d_p_')

atoms.set_calculator(calc)
print(atoms.get_potential_energy())
print(atoms.get_forces())

However, once you have instantiated the psi4 ase calculator with an atoms object you can interact with the psi4 python API as well. The psi4 API is just an attribute of the psi4 ase calculator:

calc.psi4.frequency('scf/cc-pvdz', molecule=calc.molecule,
                    return_wfn=True, dertype=1)

This is not required though, as psi4 will act like any other ase calculator.

It should be noted that the method argument supports non-DFT methods (such as coupled cluster ccsd(t)) as well. There is a great variety of quatum methods and basis sets to choose from.

Parallelization

Psi4 runs on a single thread by default. However, you may increase the number of threads by passing in the num_threads argument, which can take either “max” or integer values.

Parameters

The list of possible parameters and their defaults is shown below. See the NWChem documentation for full explanations of these different options.

keyword type default value description
label str 'psi4-calc' Label for saved files.
method str 'hf' Quantum Method or Functional
charge   None Charge
basis str 'aug-cc-pvtz' Basis set.
memory str 500MB The ammount of memory allocated to psi4
num_thread   1 The number of threads to run psi4 on
symmetry str 'c1' The symmetry of your system
PSI_SCRATCH str /tmp The scratch directory for psi4
multiplicity int` None The spin multiplicity of your system
reference str None The reference wave function. If you wish to run spin unrestricted enter “uhf”, otherwise, leave this blank.