CRITICAL TEST OF QUANTUM CLUSTER EQUILIBRIUM-THEORY - FORMIC-ACID AT B3LYP 6-31+G-ASTERISK HYBRID DENSITY-FUNCTIONAL LEVEL/

The recently developed quantum cluster equilibrium (QCE) theory is use d at the B3LYP/6-31+G level of hybrid density functional theory to ca lculate the equilibrium cluster populations for formic acid vapor, who se quantitative cluster composition was determined by Coolidge and oth ers through precise spectroscopic and vapor-density measurements over a wide range of temperature and pressure. Unlike previous experimental tests of QCE theory in which T,P-dependent cluster polpulations enter only indirectly (e.g., through convolutions with calculated cluster s pectroscopic properties, yielding comparison with inhomogeneously brao dened ''average'' spectral features), the present comparison is able t o directly test the QCE population distributions. Nearly exact agreeme nt between theory and experiment is found over the entire span of meas urements, confirming that QCE theory satisfactorily predicts the T,P-d ependent concentrations of individual cluster species, as well as the cruder population-weighted averages that enter less-sensitive comparis ons with experiment. (C) 1998 American Institute of Physics.