Numerical Calculations by the Hellmann—Feynman Procedure

Simple calculations of atomic forces and of force constants in homonuclear diatomics confirm the unreliability of the Hellmann—Feynman procedure for calculating molecular forces unless very accurate one-electron densities are used. Electronic forces are extremely sensitive to polarization and to small density changes near atomic centers, so that in general they cannot be used alone to determine accurately a totally a priori one-electron density. They may, however, be useful in testing the features of the density obtained from a variational wavefunction.