Ab initiostretching force constants for HCCH, FCCH and C1CCH

Abstract
Harmonic and anharmonic stretching force constants have been calculated for HCCH, FCCH and C1CCH from ab initio SCF wavefunctions by Pulay's force method. The ab initio harmonic and diagonal anharmonic force constants show good agreement with a recent experimental quartic force field for acetylene, while most of the force constants for FCCH and C1CCH stand as predictions. All the off-diagonal force constants (in internal valence coordinates) are calculated to be very small and thus of practically no influence on observable quantities. From systematic errors in the calculations, estimates are given for the CH and CC equilibrium bond lengths of FCCH and C1CCH.