Tests of quantum cluster equilibrium (QCE)-based computational methods for describing formic acid clustering

Quantum cluster equilibrium (QCE) theory was used to calculate the cluster populations of gaseous formic acid for a wide range of temperatures and pressures where experimental vapour density measurements are available for precise comparisons with various levels of ab initio and density functional theory. Three different theoretical methods (HF, MP2 and B3LYP) and three basis sets (3-21G, 6-31+G* and 6-311++G**) were studied. The agreement with experiment was found to depend strongly on both the level of theory and the basis set, but as expected, the agreement increases for MP2 and B3LYP which include electron correlation effects. For the basis sets, the agreement also depended on whether the energies were corrected for BSSE, this 'correction' being necessary in small basis sets but increasingly problematic as the basis set is extended. The best agreement with experiment was found using the 6-31+G* basis without correcting for BSSE, which resulted in relative percent deviations of 22% for HF, 2.0% for B3LYP and -0.4% for MP2.