Initio Density Functional Theory Study Research Articles

High level of ab initio and hybrid density functional theory study of electron affinities for some multi-spin diatomic molecules

The quadratic complete basis set ab initio computational approach and the hybrid density functional theory method were used to evaluate electron affinities of multi-spin diatomic molecules. The generalization stating that the hybrid density functional theory method is inappropriate for negative molecules was rejected, and the overestimation of the correlation effect was suggested as the cause of the high electron affinities.

Energy bands and bond alternation potential in poly(para-phenylene vinylene): a comparative ab initio quantum chemical and density functional theory study

We present calculations of the total energy per unit cell for different bond alternations of the CC bonds bridging the distance between two aromatic rings in poly(para-phenylene vinylene) (PPV), using two different parametrizations of the energy functional in the local density approximation (LDA) and the ab initio Hartree-Fock (HF) method. For the application of correlation corrections to the HF results the system is already too large. We find that even simple LDA methods are reliable alternatives to the ab initio HF method for the calculation of potential surfaces in polymers with large unit cells. The results in turn can be used to determine parameters for model Hamiltonians necessary for theoretical studies of the dynamics of nonlinear quasiparticles in the polymers. We further present the LDA band structures of PPV together with their HF and correlation (many body perturbation theory of 2nd order in Møller-Plesset partitioning, MP2) corrected counterparts. We find that the fundamental gap obtained is too large both with HF and with the correlation corrected band structure compared to experiment. However, we use only a modest correlation method and a small basis set, which already brings us to the limits of the computers available to us. The LDA gaps on the other hand are too small which, however, could be corrected with the help of self interaction corrections. None of the latter methods would lead to exceedingly large computation times.

Energy bands and bond alternation potential in poly(para-phenylene vinylene): a comparative ab initio quantum chemical and density functional theory study

We present calculations of the total energy per unit cell for different bond alternations of the CC bonds bridging the distance between two aromatic rings in poly(para-phenylene vinylene) (PPV), using two different parametrizations of the energy functional in the local density approximation (LDA) and the ab initio Hartree-Fock (HF) method. For the application of correlation corrections to the HF results the system is already too large. We find that even simple LDA methods are reliable alternatives to the ab initio HF method for the calculation of potential surfaces in polymers with large unit cells. The results in turn can be used to determine parameters for model Hamiltonians necessary for theoretical studies of the dynamics of nonlinear quasiparticles in the polymers. We further present the LDA band structures of PPV together with their HF and correlation (many body perturbation theory of 2 nd order in Møller-Plesset partitioning, MP2) corrected counterparts. We find that the fundamental gap obtained is too large both with HF and with the correlation corrected band structure compared to experiment. However, we use only a modest correlation method and a small basis set, which already brings us to the limits of the computers available to us. The LDA gaps on the other hand are too small which, however, could be corrected with the help of self interaction corrections. None of the latter methods would lead to exceedingly large computation times.

An ab initio study of the conformational energy map of acetylcholine

An ab initio density functional theory study is reported of the conformational energy map of acetylcholine, with respect to the two central dihedral angles of the molecule. The acetylcholine molecule pays a central role in neurotransmission and has been studied widely using semi-empirical computational modelling. The ab initio results are compared with a number of previous investigations and with experiment. The ab initio data indicate that the most stable conformation of acetylcholine is the trans, gauche arrangement of the central dihedral angles. Furthermore, Mulliken population analysis of the electronic structure of the molecule in this conformation indicates that the positive charge of the molecule is spread over the exterior of the cationic head of the molecule.

Quadratic complete basis set ab initio and hybrid density functional theory studies of the stability of HNC, HCN, H2NCH and HNCH2, their isomerizations, and the hydrogen insertion reactions for HCN and HNC

B. S. Jursic, J. Chem. Soc., Faraday Trans., 1997, 93, 2355 DOI: 10.1039/A701165B

Ab initio and hybrid density functional theory studies of the forward and reverse barriers for the C2H4 + H → C 2H5 reaction

Ab initio and density functional theory (DFT) methods have been used to compute the potential energy surface of the C2H4 + H → C 2H5 reaction. It was demonstrated that the computation of the forward reaction barrier is a very difficult problem. Even high level ab initio methods, such as MP2 and QCISD, had problems coming close to the experimental value. It was demonstrated that the G2 computational method predicts a very accurate energy profile for the reaction. DFT methods have a considerable problem in finding and optimizing the transition state structure. Many of these produced negative activation barriers. It was suggested that DFT methods overestimate the total energy of the hydrogen radical. Because of this effect, the heat of the reaction and the barrier of the reaction are too low. If this problem is solved, the energies generated by hybrid DFT methods for reactions which involve the hydrogen radical should be very close to experimental values.

Infrared spectra of thiouracils: experimental matrix isolation and ab initio Hartree-Fock, post-Hartree-Fock and density functional theory studies

The experimental infrared absorption spectra of 2-thiouracil, 4-thiouracil and 2,4-dithiouracil isolated in low-temperature argon matrices are compared with the spectra of these compounds computed by the density functional theory with the combined Becke3-LYP exchange-correlation energy functional (DFT(B3-LYP)) and conventional ab initio methods (Hartree-Fock (HF), MP2) using the standard 6-31G(d,p) basis set. The vibrational spectra computed at the DFT(B3-LYP)/6-31G(d,p) and HF/6-31G(d,p) levels of theory were found to reproduce well the experimental IR spectra. The results of MP2/6-31G(d,p) calculations predicted the normal modes due to out-of-plane vibrations somewhat less accurately. An assignment of the bands observed in the infrared spectra of the three thiouracils was performed, based on the comparison with the spectra calculated at the DFT(B3-LYP)/6-31G(d,p) level.