Question

What is a forcefield? (CNDO utilizes a specific type of forcefield.) How does the forcefield that is used in the computational method fit in with the idea of solving the Schrodinger equation? Does the selection of the forcefield matter? Hint: CNDO stands for Complete Neglect of Differential Overlap

Answer 1

Forcefield (ff) is the set of parameters, used to simulate a given system in molecular mechanics. these set of system includes, set of bond lengths and force constants of bond, set of dihedral angles and equations for potentials of dihedral angle, set of angles and force constants for each vibrations/bending/scissoring motions.

How it is used in solving Schrodinger equation:

we know force = F = - dU/dr      ( where U = potential energy and r = coordinates of particle)

from the above equation we can calculate U(r) and put it in Schrodinger equation as potential energy term.

There are many approximation introduced in Molecular dynamics by using forcefields to reduce computational efforts, because one of the major job of these forcefield is to convert some of the QM termss into classical mechanics terms which requires very less efforts.

Yes, Different force field gave us different results for a particular problem.

This is so because:

1) different forcefield may have different parameters for same systems also (by system we mean bond, bond angle, dihedral angle)

2) their are different approximations and models for solvations and different water models in different forcefields.

3) There are different approximations like implicit hydrogen and explicit hydrogen approximations in different forcefields.

4) In implicit solvation, even the equation for dielectric constant is different in different forcefields.

So all these 4 points ( and many other points, which I'm unable to explain here) makes each forcefield different from each others.