Maria Luz Sánchez

Multi-coefficient Gaussian-3 method for calculating potential energy surfaces

Abstract We propose a multi-coefficient modification (MCG3) of the Gaussian-3 (G3) electronic structure method that is suitable for calculating continuous potential energy surfaces. We tested it for atomization energies and found that it improves the accuracy by 8% as compared to G3 and reduces the cost of single-point energy calculations by 50%. The method should be useful for calculating bond energies, potential energy surfaces, and thermochemical data of molecules.

Infinite basis limits in electronic structure theory

We have developed a database of 29 molecules for which we have estimated the complete-one-electron-basis-set limit of the zero-point-exclusive atomization energy for five levels of electronic structure theory: Hartree–Fock (HF) theory, Mo/ller–Plesset second- and fourth-order perturbation theory, coupled cluster theory based on single and double excitations (CCSD), and CCSD plus a quasiperturbative treatment of triple excitations [CCSD(T)], all at a single set of standard geometries. Convergence checks indicate that the estimates are within a few tenths of a kcal/mol of the n=infinity limit of the cc-pVnZ basis set sequence. This data is then used to obtain optimized power-law exponents for extrapolating to the basis-set-limit from correlation-consistent polarized valence double and triple zeta (cc-pVDZ and cc-pVTZ) basis sets. This allows one to get thermochemical accuracy comparable to polarized quadruple or quintuple zeta (cc-pVQZ or cc-pV5Z) basis sets with a cost very comparable to polarized triple z...

Quantum mechanical tunneling in methylamine dehydrogenase

We report a calculation for a trideuteration kinetic isotope effect (KIE) for the proton transfer step in the oxidation of methylamine by the quinoprotein methylamine dehydrogenase (MADH). The potential field includes 11 025 atoms, and the dynamics are based on a quantum mechanical/molecular mechanical (QM/MM) direct dynamics simulation and canonical variational transition state theory with small-curvature multidimensional tunneling contributions. About 1% of the reaction occurs by overbarrier processes, with the rest due to tunneling, and the calculated KIE is reduced to 5.9 when we omit tunneling. This provides the most striking evidence yet for the contribution of tunneling processes to enzymatic reactions at physiological temperatures.

The Gaussian-2 method with proper dissociation, improved accuracy, and less cost

The Gaussian-2 method (G2) is modified by deleting the empirical high-level correction and instead using empirical coefficients to extrapolate to full configuration interaction and an infinite basis set. The resulting method, called multicoefficient Gaussian-2 (MCG2) is less expensive than G2 but a factor of 1.7 more accurate for molecules composed of H and first-row atoms.

Erratum to: “Quantum mechanical tunneling in methylamine dehydrogenase” [Chem. Phys. Lett. 347 (2001) 512–518]

We report a calculation for a trideuteration kinetic isotope effect (KIE) for the proton transfer step in the oxidation of methylamine by the quinoprotein methylamine dehydrogenase (MADH). The potential field includes 11 025 atoms, and the dynamics are based on a quantum mechanical/molecular mechanical (QM/MM) dynamics simulation and ensemble-averaged canonical variational transition state theory with small-curvature multidimensional tunneling contributions. About 1% of the reaction occurs by overbarrier processes, with the rest due to tunneling. We compute a KIE of 18.3, in good accord with experiment (17.2), but the calculated KIE is reduced to 5.9 when we omit tunneling. This provides the most striking evidence yet for the contribution of tunneling processes to enzymatic reactions at physiological temperatures.