Check whether a simple molecule is Raman-active or IR-active using PySCF

Question

I am trying to write a python code that checks the dipole moment and the polarisability of a certain molecule in order to determine if it will be Raman-active or IR-active.

Raman-active molecules are those with a change in the polarisability (so net dipole moment is zero) and IR-active are those with a change in the dipole moment (so net dipole moment is different from zero).

I was trying to use PySCF as it has an attribute for the dipole moment. However, I don't exactly understand the output that PySCF provides. Checking the dipole moment for $\ce{H2O}$ and $\ce{CO2}$ give me the same results.

The idea is that I should get a number (net dipole moment) and I would have to check if it is zero or not. Does someone know how I can do it?

This is the code I have tried:

from pyscf import gto, scf

# Define the H2O molecule
mol = gto.M(atom='O 0 0 0; H 0 1 0; H 0 0 1', basis='sto-3g')

# Perform a RHF calculation for the ground state
mf = scf.RHF(mol)
mf.kernel()

# Extract the dipole moment for the ground state
ground_state_dipole = mf.dip_moment()

# Perform a RHF calculation for the perturbed state
mf = scf.RHF(mol)
mf.charge = -1
mf.kernel()

# Extract the dipole moment for the perturbed state
perturbed_state_dipole = mf.dip_moment()

# Calculate the change in dipole moment
dipole_diff = perturbed_state_dipole - ground_state_dipole
print("Change in dipole moment of H2O:", dipole_diff)

Answer 1

The dipole moment that is calculated by mf.dip_moment() is a vector where the three values returned (µx, µy, and µz) which are the x, y and z components of dipole moment.

So the resultant magnitude of the vector is :

So in terms of code, it would be:

dipole_moment = mf.dip_moment()
total = 0
for component in dipole_moment:
    total+= component**2

resultant_dipole_moment = total**0.5

You can then use this resultant_dipole_moment to figure out whether the compound is raman active or IR active