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 in chemistry is a set of parameter and equations for use in molecular mechanics simulations

Symmetry force field calculations have been performed for the planar vibrations of urea using CNDO/Force method.

Yes the selection of force field matters as

The CNDO/Force calculations predict well the signs and the magnitudes of bending and interaction force constants; the stretching force constants are found to be higher in magnitude. The bending and interaction constants obtained from these calculations and the stretching force constants obtained from the literature are considered for the initial force field. For example, Using the observed frequencies for urea and its isotopic analogues, urea-D4, urea-15N2, urea-15N2D4 and urea-18O in the solid as well as in the solution phases, the force field is refined by carrying out iterations over the diagonal force constants. In the final stages of the refinement iterations are carried out over all the force constants keeping the signs of the interaction constants unchanged. It is found that the agreement between the calculated and the observed frequencies is excellent. The final force fields in terms of symmetry as well as redundancy free internal valence coordinates are reported.