The IR spectra of nonionized glycine and its deuterated derivatives isolated in the low-temperature argon matrices have been measured, and for the first time the infrared spectral characteristics of the three most stable conformers have been determined and assigned. Correlated level ab initio and density functional theory (DFT) calculations of IR frequencies and intensities with extended basis sets were performed and their results were employed to separate the bands of the glycine conformers in the experimental spectra and to assist the assignment. The intramolecular interconversion, conformer III w conformer I, which is observed in the matrices at temperatures higher than 13 K, was found to cause a significant decrease of the band intensities of conformer III in the spectra. This phenomenon was used to distinguish the vibrational bands of this conformer from the bands of the other conformers. The reliability of the Møller-Plesset second-order perturbation theory (MP2) method and the DFT method with the three-parameter density functional, Becke3LYP, in the prediction of the IR spectra of the nonionized glycine conformers was examined. We found that the DFT/Becke3LYP method with aug-cc-pVDZ basis set yields vibrational frequencies of the glycine conformers very similar to the MP2 results. Both DFT and MP2 results are in the excellent agreement with the experimental data.

The variational method complemented with the use of explicitly correlated Gaussian basis functions is one of the most powerful approaches currently used for calculating the properties of few-body systems. Despite its conceptual simplicity, the method offers great flexibility, high accuracy, and can be used to study diverse quantum systems, ranging from small atoms and molecules to light nuclei, hadrons, quantum dots, and Efimov systems. The basic theoretical foundations are discussed, recent advances in the applications of explicitly correlated Gaussians in physics and chemistry are reviewed, and the strengths and weaknesses of the explicitly correlated Gaussians approach are compared with other few-body techniques.

Two conformers of the nonionized α-alanine and its isotopomer N,N,O-d 3-alanine have been observed in low-temperature Ar matrixes. Their infrared spectra have been analyzed and assigned using DFT/B3LYP/aug-cc-pVDZ and MP2/aug-cc-pVDZ geometry and frequency theoretical calculations. Two different intramolecular H-bonds, bifurcated NH2···OC and N···H−O, were found in the observed α-alanine conformers, I and IIa. We found that the DFT/B3LYP/aug-cc-pVDZ method yields vibrational frequencies of the α-alanine conformers in excellent agreement with the experimental data.

A new state-selective multireference (MR) coupled-cluster (CC) method exploiting the single-reference (SR) particle-hole formalism is described. It is an extension of a simple two-reference formalism, which we presented in our earlier paper [N. Oliphant and L. Adamowicz, J. Chem. Phys. 94, 1229 (1991)], and a rigorous formulation of another method of ours, which we obtained as an approximation of the SRCC approach truncated at triple excitations (SRCCSDT) [N. Oliphant and L. Adamowicz, J. Chem. Phys. 96, 3739 (1992)]. The size extensivity of the resulting correlation energies is achieved by employing a SRCC-like ansatz for the multideterminantal wave function. General considerations are supplemented by suggesting a hierarchy of approximate schemes, with the MRCCSD approach (MRCC approach truncated at double excitations from the reference determinants) representing the most important one. Our state-selective MRCCSD theory emerges through a suitable selection of the most essential cluster components appearing in the full SRCCSDTQ method (SRCC method truncated at quadruple excitations), when the latter is applied to quasidegenerate states. The complete set of equations describing our MRCCSD formalism is presented and the possibility of the recursive intermediate factorization [S. A. Kucharski and R. J. Bartlett, Theor. Chim. Acta 80, 387 (1991)] of our approach, leading to an efficient computer algorithm, is discussed.

The molecular structure of pyruvic acid was investigated by matrix isolation FTIR spectroscopy, density functional theory (DFT), and ab initio calculations performed at the RHF, MP2, MP4(SDQ), and CCSD(T) levels of theory with the aug-cc-pVDZ basis set. In these calculations, the geometries of the three lowest energy conformers of pyruvic acid were fully optimized at the DFT/B3LYP/aug-cc-pVDZ and MP2/aug-ccpVDZ levels. Additionally, the relative energies of the conformers were calculated at the MP4, CCSD, and CCSD(T) levels. Harmonic frequencies and IR intensities were then calculated for these three conformers and were used to account for the zero point vibrational energy corrections and to assist the assignment of the observed bands to the different forms. We found that two conformers are present in the Ar matrix, and both forms exhibit a planar framework with the carbonyl bonds in a trans arrangement but differ in the orientation of the hydroxyl hydrogen. By varying the temperature of the pyruvic acid vapor prior to matrix deposition we were able to separate the bands due to the two conformers and measure their enthalpy difference. The spectral signature of the second pyruvic acid conformer has been identified for the first time. Experimental enthalpy difference between the two most stable conformers was found to be 8.7((15%) kJ mol -1 , which is in good agreement with the theoretical result.

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A multireference coupled-cluster singles and doubles method utilizing two reference determinants which differ by a two electron excitation is proposed. One of these determinants is selected as the formal reference determinant. The proposed method includes single-reference coupled-cluster equations truncated after quadruples. These equations are graphically derived using Feynman diagrams. The appropriate restrictions are then placed on the triple and quadruple amplitudes to allow only those amplitudes which correspond to single and double excitations from the second reference determinant.