The structures, interaction energies, vibrational analysis, and electronic properties for different urea-hydrogen peroxide (UHP) 1:1 complexes have been studied. Density functional theory (DFT) using the B3LYP hybrid exchange-correlation functional was employed to characterize five new cyclic structures in addition to the experimental one. The basis sets used were Dunning’s correlation consiste...

Neutral and anionic 13-atom aluminum clusters are studied with high-level, fully ab initio methods: second-order perturbation theory (MP2) and coupled cluster theory with singles, doubles, and perturbative triples (CCSD(T)). Energies and vibrational frequencies are reported for icosahedral and decahedral isomers, and are compared with density functional theory results. At the MP2 level of theor...

A new approximation for post-Hartree Fock (HF) methods is presented applying ten-sor decomposition techniques in the canonical product tensor format. In this ansatz, multidimensional tensors like integrals or wavefunction parameters are processed as an expansion of one-dimensional representing vectors. This approach has the potential to decrease the computational effort and the storage requirem...

A number of approaches are proposed and assessed to reduce the frozen natural orbital (FNO) truncation error coupled-cluster singles doubles with perturbative triples [CCSD(T)] energies. The diagrammatic energy decomposition method Irmler Grüneis [J. Chem. Phys. 151, 104107 (2019)] is extended FNO correction particle-particle ladder (PPL) term in case closed- open-shell molecular systems. appro...

The reaction of cyclobutane-1,2-dione with hydroxide was studied by a variety of ab initio (MP2, SCS-MP2, CCSD(T), CEPA/1) and density functional (M06-2X) methods. Three possible reaction paths of the initially formed tetrahedral adduct leading to either 1-hydroxycyclopropane-1-carboxylate (benzilic acid type rearrangement, path A), α-oxobutanoate (path B) or γ-oxobutanoate (path C) were consid...

Two electron correlation theories, second-order Møller-Plesset perturbation (MP2), and density functional (DFT) methods have been adopted to obtain fully optimized structures of styrene, trans-stilbene, and cisstilbene. Full geometry optimizations with MP2 shows that the nonplanar conformations of styrene and trans-stilbene are preferred by 0.24 (styrene) and 0.80 kcal/mol (trans-stilbene), res...

There has been much interest in cost-free improvements to second-order Møller-Plesset perturbation theory (MP2) via scaling the same- and opposite-spin components of the correlation energy (spin-component scaled MP2). By scaling the same- and opposite-spin components of the double excitation correlation energy from the coupled-cluster of single and double excitations (CCSD) method, similar impr...

We develop an alternative formulation in the energy-domain to calculate the second order Møller-Plesset (MP2) perturbation energies. The approach is based on repeatedly choosing four random energies using a nonseparable guiding function, filtering four random orbitals at these energies, and averaging the resulting Coulomb matrix elements to obtain a statistical estimate of the MP2 correlation e...

We have studied how the calculation of electronic and vibrational contributions to nonlinear optical properties of three representative p-conjugated organic molecules is affected by the choice of basis set and the inclusion of electron correlation effects. The 6-31G basis does not always provide even qualitative accuracy. For semiquantitative accuracy a 6-311G(d) basis is sufficient. Although, ...

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