| Abstract
| - Three new multicoefficient correlation methods (MCCMs) called BMC-QCISD, BMC-CCSD, and BMC-CCSD-C are optimized against 274 data that include atomization energies, electron affinities, ionizationpotentials, and reaction barrier heights. A new basis set called 6-31B(d) is developed and used as part of thenew methods. BMC-QCISD has mean unsigned errors in calculating atomization energies per bond and barrierheights of 0.49 and 0.80 kcal/mol, respectively. BMC-CCSD has mean unsigned errors of 0.42 and 0.71kcal/mol for the same two quantities. BMC-CCSD-C is an equally effective variant of BMC-CCSD thatemploys Cartesian rather than spherical harmonic basis sets. The mean unsigned error of BMC-CCSD orBMC-CCSD-C for atomization energies, barrier heights, ionization potentials, and electron affinities is 22%lower than G3SX(MP2) at an order of magnitude less cost for gradients for molecules with 9−13 atoms, andit scales better (N6 vs N,7 where N is the number of atoms) when the size of the molecule is increased.
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