• YAKHAK HOEJI
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• v.33 no.2
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• pp.87-100
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• 1989

Salicylic acids as anti-inflammatory agents were analyzed by ab initio, quantum chemical methods to study the possible modes of binding to the receptor. As the result of multiple regression analysis of reactivity indices and interpretation of normalized frontier orbital charges of drugs, potency seems to be related to energy of HOMO and LUMO at the 5 position of benzene ring, and in the 5-phenyl substituted case, the para position of substituting ring is important. The binding occurs first at the positive site of its receptor. The charge density exhibited by the frontier orbitals suggests that charge moves from receptor site to carboxyl group. The electrostatic orientation effect makes an important contribution to the binding of the active molecules to their receptors. Also the electrostatic potential model may be able to rationalize the source of activity or inactivity of the drugs under investigation.

• Bulletin of the Korean Chemical Society
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• v.34 no.1
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• pp.179-187
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• 2013

We describe newly developed software named KPACK for relativistic electronic structure computation of molecules containing heavy elements that enables the two-component ab initio calculations in Kramers restricted and unrestricted formalisms in the framework of the relativistic effective core potential (RECP). The spin-orbit coupling as relativistic effect enters into the calculation at the Hartree-Fock (HF) stage and hence, is treated in a variational manner to generate two-component molecular spinors as one-electron wavefunctions for use in the correlated methods. As correlated methods, KPACK currently provides the two-component second-order M${\o}$ller-Plesset perturbation theory (MP2), configuration interaction (CI) and complete-active-space self-consistent field (CASSCF) methods. Test calculations were performed for the ground states of group-14 elements, for which the spin-orbit coupling greatly influences the determination of term symbols. A categorization of three procedures is suggested for the two-component methods on the basis of spin-orbit coupling manifested in the HF level.

• Journal of the Korean Chemical Society
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• v.42 no.1
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• pp.9-15
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• 1998

The geometrical parameters, vibrational frequencies, and IR intensities of dimethyldioxirane had been investigated using high level ab initio quantum mechanical methods with various basis sets. The polarization function decreases C-O and C-C bond distances significantly and the electron correlation effect increases those bond lengths slightly, while other bond lengths and bond angles are relatively stable for basis set size and correlation effect. The experimental and other theoretical vibrational frequencies and IR intensities of dimethyldioxirane will be compared and discussed with our high level theoretical predictions.

• Journal of the Korean Chemical Society
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• v.60 no.6
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• pp.410-414
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• 2016

The anharmonic frequency of a local OH stretching mode of a water monomer and dimer was calculated using various levels of density functional theory. The quantum chemical potential energy curves as a function of the OH bond distance were calculated, and they were fitted with the Morse potential function to analytically obtain the fundamental transition frequency. By comparing those values with the frequencies similarly calculated using an ab initio quantum chemical method, the coupled cluster theory including both single and double excitations with the perturbative inclusion of triple excitation in the complete basis limit, the accuracy of various density functional methods in the calculation of anharmonic vibration frequency of water molecules was assessed. For a water monomer, X3LYP and B3LYP methods give the best accuracy, whereas for a water dimer, B972, LCBLYP, ${\omega}B97X$, ${\omega}B97$ methods show the best performance.

• Bulletin of the Korean Chemical Society
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• v.21 no.5
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• pp.510-514
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• 2000

Molecular hydrogen dimer, ($H_2)_2$ is a weakly bound van der Waals complex. The configuration of two hydrogen molecules and the potential well structure of the dimer have been the subjects of various studies among chemists and astrophysicists. In this study, we used DFT, MCG2, and MCG3 methods to determine the structure and energy of the molecular hydrogen dimer. We compared the results with previously reported ab initio method results. The ab initio results were also recalculated for comparison. All optimized geometries obtained from the MP2 and DFT methods are T-shaped. The H-H bond lengths for the dimer are almost the same as those of monomer. The center-to-center distance depeds on the levels of theory and the size of the basis sets. The bond lengths of the $H_2$ molecule from the MCG2 and MCG3 methods are shown to be in excellent agreement with the experimental value. The geometry of optimized dimer is T-shaped, and the well depths for the dimerization potential are very small, being 23 $cm-^1$ and 27 $cm-^1$ at the MCG2 and MCG3 levels, respectively. In general the MP2 level of theory predicts stronger van der Waals than the DFT, and agrees better with the MCG2 and MCG3 theories.

• Bulletin of the Korean Chemical Society
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• v.14 no.4
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• pp.476-480
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• 1993

The ab initio calculations were performed on ClO and $ClO^+$ using the configuration interaction and M${\phi}$ller-Plesset methods of several different levels of approximation. Three different basis sets, 66 contracted Gaussian-type orbitals,6-31$G^*$ and 6-311$G^*$, were employed in this calculation. The results of calculation were compared with the experimental values of ClO. The values from the calculation with 66cGTO basis set gave excellent agreement with the experimental values. The spectroscopic constants of $ClO^+$ were also predicted.

• Bulletin of the Korean Chemical Society
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• v.31 no.2
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• pp.457-460
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• 2010

The geometrical structures and energetics of positively doubly charged fullerene dimer $(C_{60})_2{^{2+}}$ conformers were studied using semiempirical PM3 and MNDO, Hartree-Fock (HF), and Hybrid B3LYP density functional methods. The shape of the HOMO-LUMO for the three conformers was also analyzed. The gauche conformer was the most stable of the three conformers. The anti conformer was more stable than the syn conformer.

• Bulletin of the Korean Chemical Society
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• v.30 no.12
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• pp.2973-2978
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• 2009

The decomposition reaction mechanism of $CH_3CF_2O$ radical formed from hydroflurocarbon, $CH_3CHF_2$ (HFC-152a) in the atmosphere has been investigated using ab-initio quantum mechanical methods. The geometries of the reactant, products and transition states involved in the decomposition pathways have been optimized and characterized at DFT-B3LYP and MP2 levels of theories using 6-311++G(d,p) basis set. Calculations have been carried out to observe the effect of basis sets on the optimized geometries of species involved. Single point energy calculations have been performed at QCISD(T) and CCSD(T) level of theories. Out of the two prominent decomposition channels considered viz., C-C bond scission and F-elimination, C-C bond scission is found to be the dominant path involving a barrier height of 12.3 kcal/mol whereas the F-elimination path involves that of a 28.0 kcal/mol. Using transition-state theory, rate constant for the most dominant decomposition pathway viz., C-C bond scission is calculated at 298 K and found to be 1.3 ${\times}$ 10$^4s{-1}$. Transition states are searched on the potential energy surfaces involving both decomposition channels and each of the transition states are characterized. The existence of transition states on the corresponding potential energy surface are ascertained by performing Intrinsic Reaction Coordinate (IRC) calculation.

• Journal of Photoscience
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• v.7 no.2
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• pp.67-71
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• 2000

A quantum-chemical investigation on the conformations and electronic properties of trans(diphenyl-diheterocyclic) ethenes(t-PHEs) as building block for fully $\pi$-conjuated polymer are performed in order to display the effects of heterocyclic ring substitution. Structures for the molecules, t-PHEs were fully optimized by using semiempirical AM1, PM3 methods, and ab initio HF methods, with 6-31G basic set. The potential energy curves with respect to the change of single are obtained by using ab initio HF/6-31G basic set. The curves are not similar shapes in the molecules with respect to heterocyclic rings. It is shown that the steric repulsion interactions between phenyl ring and heterocyclic ring are subjected to different type with the respect to each heterocyclic ring. Electronic properties of the molecules were molecules were obtained by applying the optimized structures and selected geometries to the extended Huckel method. To investigate the change of HOMO-LUMO gap with respedt to the torsion angle, we select the optimized structures. By using the results, the dependency of conjugation for the energy gaps is analyzed. For t-PHE the energy gap increase up to 0.52 eV compared with its planar structure. In the cases of t-PHE and t-PHE, the energy gap increase by 1.29 and 1.15 eV, respectively, compared with its planar structure.

• Bulletin of the Korean Chemical Society
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• v.32 no.10
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• pp.3634-3640
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• 2011

We have performed quantum mechanical calculations to study the geometries and binding energies of biologically important, cyclic hydrogen-bonded complexes, such as formic acid + $H_2O$, formamidine + $H_2O$, formamide + $H_2O$, formic acid dimer, formamidine dimer, formamide dimer, formic acid + formamide, formic acid + formamidine, formamide + formamidine, and barrier heights for the double proton transfer in these complexes. Various ab initio, density functional theory, multilevel methods have been used. Geometries and energies depend very much on the level of theory. In particular, the transition state symmetry of the proton transfer in formamidine dimer varies greatly depending on the level of theory, so very high level of theory must be used to get any reasonable results.