ИСТИНА

The estimates of geometrical parameters and energy difference between points on the potential energy surface (PES) (the conformers energy differences, barriers of conformational transitions) small organic molecules in the ground and lowest excited electronic states were systematically investigated by means of various quantum-chemical techniques (MP2, CCSD(T), CASSCF, CASPT2, etc. within various AO basis sets). Also, method VFPA was used. Extrapolative VFPA (Valence Focal-Point Analysis) technique [1] is based on a series of quantum-chemical methods with a successive increase of account for electron correlation (in the present work: HF→MP2→MP3→MP4→… and HF→MP2→CCSD→CCSD(T) →…) and on series of correlation-consistent basis sets (aug)-cc-p(C)XZ (X=D,T,Q,5,6). Such approach allows performing two-dimensional (method/basis) extrapolation to the Full Configuration Interaction method within a Complete Basis Set. Obtained values are corrected for zero point vibrational energy, relativistic corrections, core-valence correlation, and non-Born-Oppenheimer effects. Results were compared with experimental data as well as with estimations of other high-level (QCISD и CCSD(T)) quantum-chemical methods and the results of “semi-empirical” approaches based on ab initio and (or) DFT calculations designed for evaluation of thermodynamic parameters (G2, G3 and their modifications).