# 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

Basic calculator implementation.

restart: str

Prefix for restart file. May contain a directory. Default is None: don’t restart.

Ignore broken or missing restart file. By default, it is an error if the restart file is missing or broken.

directory: str

Working directory in which to read and write files and perform calculations.

label: str

Name used for all files. Not supported by all calculators. May contain a directory, but please use the directory parameter for that instead.

atoms: Atoms object

Optional Atoms object to which the calculator will be attached. When restarting, atoms will get its positions and unit-cell updated from file.

## 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'

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.