ACHIEVING RELIABILITY OF CALCULATIONS FOR FLAT POTENTIAL SURFACES IN DENSITY-FUNCTIONAL THEORY - THE CASE OF AL-4 AND AL-4(+1)

Harmonic frequencies obtained by finite-differences from nonlocal dens ity functional calculations are presented for the ground states of Al- 4 (neutral and cationic). The effect of varying the step size used in the finite-difference evaluation and the influence of the density conv ergence threshold are discussed. Potential energy curves along the mos t important normal coordinates are shown. With these results, we found that for Al-4 the square and the rhombus minima are almost degenerate with each other, while for Al-4(+1), the rhombus is more stable and t he square is a transition state. (C) 1997 John Wiley & Sons, Inc.