Adapted Cluster-configuration Interaction Research Articles

Investigation on the electronic structures of thiamine and related compounds: Free base or salt?

We report the simulated photoelectron spectra of thiamine free base, thiamine and thiamine diphosphate (ThDP) in the gas phase. The structures of eighty conformers of thiamine (vitamin B1) were optimized by DFT method at the B3LYP/6-31+G(d) level. Then population ratios (PRS) with Boltzmann distribution equation were calculated using Gibbs free energy and the stable conformers were obtained. The ionization energies of these conformers were determined by two high-level computational methods. General-R-symmetry adapted cluster-configuration interaction (general-R-SAC-CI) method and electron-propagator theory (EPT) method were used to obtain vertical valence ionization energies (IP). To characterize the nature of ionization processes about different spectral bands, the natural bonding orbital (NBO) method was used to obtain orbital electron densities. The electron correlation was taken into account with SAC-CI method and MOs shapes and ionization assignments given for each spectral band. Results for ThDP show that spectra of two conformers are different: oxygen lone pair ionizations appear at higher ionization energies in the spectrum of conformer II while giving rise to the first ionization band in conformer I. The validity of Koopmans’s approximation was considered for each ionization band and the role of electron correlation was attributed to breaking down of this approximation. The implications of electronic structure results for biological activity of thiamin are discussed. Comparison with the earlier work using aerosol thermodesorption method suggests that free base was present in their photoionization spectra in addition to the deprotonated cation.

Theoretical Study on the Low-Lying Electronic States of He2, He+2, and He++2

The equilibrium geometries, potential energy curves, spectroscopic dissociation energies of the ground and low-lying electronic states of He2, He+2 and He++2 are calculated using symmetry adapted cluster/symmetry adapted cluster-configuration interaction (SAC/SAC-CI) method with the basis sets CC-PV5Z. The corresponding dissociation limits for all states are derived based on atomic and molecular reaction statics. The analytical potential energy functions of these states are fitted with Murrell—Sorbie potential energy function from our calculation results. The spectroscopic constants Be, αe, ωe, and ωeχe of these states are calculated through the relationship between spectroscopic data and analytical energy function, which are in well agreement with the experimental data. In addition, the origin of the energy barrier in the ground state X1Σg+ of He2++ energy curve are explained using the avoided crossing rules of valence bond model.

A new fragment-based approach for calculating electronic excitation energies of large systems

We present a new fragment-based scheme to calculate the excited states of large systems without necessity of a Hartree-Fock (HF) solution of the whole system. This method is based on the implementation of the renormalized excitonic method [M. A. Hajj et al., Phys. Rev. B 72, 224412 (2005)] at ab initio level, which assumes that the excitation of the whole system can be expressed by a linear combination of various local excitations. We decomposed the whole system into several blocks and then constructed the effective Hamiltonians for the intra- and inter-block interactions with block canonical molecular orbitals instead of widely used localized molecular orbitals. Accordingly, we avoided the prerequisite HF solution and the localization procedure of the molecular orbitals in the popular local correlation methods. Test calculations were implemented for hydrogen molecule chains at the full configuration interaction, symmetry adapted cluster/symmetry adapted cluster configuration interaction, HF/configuration interaction singles (CIS) levels and more realistic polyene systems at the HF/CIS level. The calculated vertical excitation energies for lowest excited states are in reasonable accordance with those determined by the calculations of the whole systems with traditional methods, showing that our new fragment-based method can give good estimates for low-lying energy spectra of both weak and moderate interaction systems with economic computational costs.

Origin of Near-Infrared Absorption and Large Second Hyperpolarizability in Oxyallyl Diradicaloids: A Three-State Model Approach

Bis(benzofuranonyl)methanolate (BM4i4i) dye and croconate dyes (derivatives of oxyallyl molecules) in general are known to have intense transitions in the near-infrared (NIR) region, indicating small transition energies and large transition dipole moments. These molecules have been reported in the literature to have very large resonant third-order nonlinear optical (NLO) susceptibilities and molecular hyperpolarizabilities (gamma). In this work we investigate using density functional theory (DFT)/ab initio/symmetry adapted cluster-configuration interaction (SAC-CI) techniques the oxyallyl substructure and attribute the NIR transition and the NLO activity to this substructure, which is common in all these molecules. Using valence bond (VB) theory, an analysis of a three-state model of this substructure is carried out. It is seen that the mixture of an intermediate diradical character and some zwitterionic character in the molecule and a large coupling between these two VB resonance forms is responsible for large gamma values. This can be used as a design principle for increasing NLO activity in oxyallyl derivatives.

Investigation of analytic potential energy function, harmonic frequency and vibrational levels for the 23Σ+g state of spin-aligned dimer 7Li2

Using the symmetry adapted cluster/symmetry adapted cluster-configuration interaction(SAC/SAC-CI) method in Gaussian03 program package, the equilibrium geometry of the 23Σ+g state of spin-aligned dimer 7Li2 is calculated at a number of basis sets. At the same time, the single-point energy scanning calculation is also made at each basis set near the equilibrium internuclear separation obtained by the geometry optimization so as to attain the more accurate result. A disagreement between the result obtained by the geometry optimization and that obtained by the single-point energy scanning calculation is found. Our analysis shows that the result obtained by the single-point energy scanning calculation should be more reasonable. We drew the conclusion that the basis sets 6-311++G(3df,3pd), 6-311++G(2df,2pd) and 6-311++G(2df,pd) are the most suitable ones for the 23Σ+g state calculation. The complete potential energy curve is further scanned at SAC-CI/6-311++G(3df,3pd) level of theory for the state over the internuclear separation range from 2.5a0 to 37a0, then a least squares fit to the Murrell-Sorbie function is made, at last the harmonic frequency is calculated, which is in good agreement with other theoretical results. At the same time, the same calculations are made for the ground state for comparison. In addition, we have also calculated the vibrational levels and the classical turning points.