Force-constant computations in cartesian coordinates. Elimination of translational and rotational contributions

A projection method is described for elimination of spurious contributions to calculated cartesian force constants which give rise to non-zero frequencies for translational and rotational modes and which may cause errors in vibrational frequencies. Illustrative calculations for water monomer (in STO—3G) and water dimer (in 4–31G) are discussed. The preference for deuterium-bonded versus hydrogen-bonded isotopomeric water -dimer structures is demonstrated by calculations which satisfy the Teller—Redlich product rule provided that projected force constants are employed.