A STUDY OF NEGATIVE FORCE CONSTANTS - A METHOD TO OBTAIN FORCE CONSTANTS BY ELECTRONIC STRUCTURES

摘要

The potential of negative force constants appearing in the valence-force model is discussed from the viewpoint of electronic structures, Prior to discussing this, a general procedure for obtaining force constants by the first-principles calculations is given, while simpler examples are worked out, For the strongest bond-stretching force, it is almost always calculable without ambiguity by using the breathing type deformation. For non-central forces, such as angle-bending forces, there is a practical difficulty in estimation, The expansion of the adiabatic potential obtained by the first-principles calculations involves so many force constants other than the bond-stretching and angle-bending forces that the values of angle-bending force constants so obtained usually exhibit large errors, Only for more simple molecules is it possible to evaluate the noncentral forces without ambiguity, In such a case, negative values of the angle-bending force constants appear, depending on the electronic configuration, How the Jahn-Teller effect participates in negative force constants has been investigated, The Jahn-Teller effect can contribute, but not all negative forces are ascribed to this effect. (C) 1997 Academic Press. [References: 17]