Note

No recompile of the PSI program is necessary for changes made to files in $PSIDATADIR aka psi4/lib, including those described below.

Defining a Method Alias

Since quantum chemical methods in PSI4 are accessed through Python functions, and most important quantities are available as PSI variables, it is straightforward to create aliases to commonly run calculations or to define hybrid methods. The psi4/lib/python/aliases.py file is intended for editing by the user for this purpose.

As an example, the MP2.5 method is the average of MP2 and MP3. The latter is available through the arbitrary order MPn code and returns all lower energies along with it in PSI variables. The following is basic code that will compute and return the MP2.5 energy.

def run_mp2_5(name, **kwargs):

    energy('mp3', **kwargs)
    e_scf = psi4.get_variable('SCF TOTAL ENERGY')
    ce_mp2 = psi4.get_variable('MP2 CORRELATION ENERGY')
    ce_mp3 = psi4.get_variable('MP3 CORRELATION ENERGY')

    ce_mp25 = 0.5 * (ce_mp2 + ce_mp3)
    e_mp25 = e_scf + ce_mp25

    print """  MP2.5 total energy:                      %16.8f\n""" % (e_mp25)
    print """  MP2.5 correlation energy:                %16.8f\n""" % (ce_mp25)

    return e_mp25

Compare the above to the method that resides in psi4/lib/python/aliases.py. The rationale for the changes is indicated in the comments below.

def run_mp2_5(name, **kwargs):
    lowername = name.lower()  # handy variable with name keyword in lowercase
    kwargs = kwargs_lower(kwargs)  # removes case sensitivity in keyword names

    # Run detci calculation and collect conventional quantities
    energy('mp3', **kwargs)
    e_scf = psi4.get_variable('SCF TOTAL ENERGY')
    ce_mp2 = psi4.get_variable('MP2 CORRELATION ENERGY')
    ce_mp3 = psi4.get_variable('MP3 CORRELATION ENERGY')
    e_mp2 = e_scf + ce_mp2  # reform mp2 and mp3 total energies for printing
    e_mp3 = e_scf + ce_mp3

    # Compute quantities particular to MP2.5
    ce_mp25 = 0.5 * (ce_mp2 + ce_mp3)
    e_mp25 = e_scf + ce_mp25
    psi4.set_variable('MP2.5 CORRELATION ENERGY', ce_mp25)  # add new method's important results
    psi4.set_variable('MP2.5 TOTAL ENERGY', e_mp25)         #     to PSI variable repository
    psi4.set_variable('CURRENT CORRELATION ENERGY', ce_mp25)
    psi4.set_variable('CURRENT ENERGY', e_mp25)  # geometry optimizer tracks this variable, permits
                                                   #     MP2.5 finite difference optimizations
    # build string of title banner and print results
    banners = ''
    banners += """psi4.print_out('\\n')\n"""
    banners += """banner(' MP2.5 ')\n"""
    banners += """psi4.print_out('\\n')\n\n"""
    exec banners

    tables  = ''
    tables += """  SCF total energy:                        %16.8f\n""" % (e_scf)
    tables += """  MP2 total energy:                        %16.8f\n""" % (e_mp2)
    tables += """  MP2.5 total energy:                      %16.8f\n""" % (e_mp25)
    tables += """  MP3 total energy:                        %16.8f\n\n""" % (e_mp3)
    tables += """  MP2 correlation energy:                  %16.8f\n""" % (ce_mp2)
    tables += """  MP2.5 correlation energy:                %16.8f\n""" % (ce_mp25)
    tables += """  MP3 correlation energy:                  %16.8f\n""" % (ce_mp3)
    psi4.print_out(tables)  # prints nice header and table of all involved quantities to output file

    return e_mp25

One final step is necessary. At the end of the aliases.py file, add the following line.

procedures['energy']['mp2.5'] = run_mp2_5

This permits the newly defined MP2.5 method to be called in the input file with the following command.

energy('mp2.5')