Vibrational corrections to properties at arbitrary reference geometry

We demonstrate how vibrational contributions to any (static) electric prope rty may be computed with respect to an arbitrary reference geometry which, at a given level of electronic structure theory, need not correspond to the associated minimum energy geometry. Within the harmonic approximation, it is shown that the formulas for the vibrational contributions can be extende d to include a second-order corrective term, which is a function of the ene rgy gradient and the (nuclear) first derivatives of the property in questio n. Taking the BH molecule as a test case, we illustrate that the order of m agnitude of the correction increases with order of property (i.e., mu appro ximate to 10(-2) --> gamma approximate to 10(1)-10(2)), and that this value is equivalent to the difference in (pure) electronic contributions evaluat ed with respect to the optimum and nonoptimum geometries. Furthermore, we s how that for a diatomic, vibrational [zero-point vibrational average (ZPVA) and pure] contributions computed at a nonoptimum geometry may be readily c orrected to give the optimum geometry values. Thus we provide a route for o btaining total (electronic + vibrational) properties associated with a mini mum energy geometry, using information calculated at a nonoptimum geometry. (C) 2000 American Institute of Physics. [S0021-9606(00)31004-2].