Question

I have a 2-fragment molecular complex with partial charge transfer from fragment 1 to fragment 2. The binding energy is ~0.3 eV and I want to evaluate what fraction comes from electrostatic attraction. I have the wavefunction file and I was wondering if there is a way I can evaluate the electrostatic attraction between the two fragments on either Gaussian or Multiwfn.

I can think of a simple approach of performing some type of atomic charge assignment (eg using NAO) and calculating the Coulomb attraction by treating each atom as a point charge, but I was hoping to be able to account for distributed electron density in the calculation.

Answer 1

To evaluate the electrostatic attraction between the two fragments in a molecular complex using Gaussian or Multiwfn, you can calculate the interaction energy by subtracting the energy of the separated fragments from the energy of the complex. This can be achieved by performing separate calculations for the isolated fragments and the molecular complex at the same level of theory. The electrostatic contribution can then be extracted from the interaction energy.

Step-by-step explanation:

The first step is to perform calculations for the isolated fragments and the molecular complex using the same level of theory in Gaussian or another quantum chemistry software. Once you have the total energies for the fragments (E_frag1 and E_frag2) and the molecular complex (E_complex), the interaction energy (ΔE_int) is calculated as follows:

ΔE_int = E_complex - (E_frag1 + E_frag2)

This interaction energy consists of various contributions, including electrostatic, exchange, and dispersion. To isolate the electrostatic contribution, one approach is to perform a separate calculation using the charge distribution obtained from a Natural Atomic Orbital (NAO) analysis. This involves assigning atomic charges and calculating the Coulombic interaction between the point charges associated with each atom.

While Gaussian can provide information on the total interaction energy, tools like Multiwfn can be employed for more detailed analyses of charge distributions and electron density. Multiwfn allows for population analysis and visualization of charge density, aiding in a more comprehensive understanding of the electrostatic contribution to the binding energy in the molecular complex