X alpha local spin density calculations were performed for a body of 112 mo
lecules containing fluorine and second-row elements, silicon, phosphorus, s
ulphur and chlorine. The parameter alpha was treated as a variable and adap
ted to each molecule, depending upon the number and type of bonds involved.
The X alpha atomization energies are in acceptable agreement with their ex
perimental counterparts. The standard deviations, 5.6 and 6.2 kcal mol(-1)
for the X alpha(6-31G**) and X alpha(3-21G) methods, respectively, compare
well with those obtained earlier for molecules containing only first-row el
ements. Compared to the sophisticated self-consistent hybrid B3LYP model, u
sed here with a 6-311G(d,p) basis, our simplified approach permits consider
able savings of computer time. It is shown that the 3-21G orbital basis gen
erally offers a reasonable compromise between accuracy and economy. (C) 199
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