DENSITY FUNCTIONAL EFFICIENCY IN THE CALCULATIONS OF VIBRATIONAL FREQUENCIES AND MOLECULAR STRUCTURES OF b-DIKETONES

2016 ◽  
Vol 57 (1) ◽  
Keyword(s):  
Density Functional ◽  
Vibrational Frequencies ◽  
Molecular Structures ◽  
Functional Efficiency

scholarly journals Density Functional Theory Study on Conformers of Benzoylcholine Chloride

2013 ◽  
Vol 2013 ◽  
pp. 1-10 ◽  
Author(s):  
Mustafa Karakaya ◽  
Fatih Ucun ◽  
Ahmet Tokatlı
Keyword(s):  
Density Functional Theory ◽  
Ground State ◽  
Density Functional ◽  
Atomic Orbital ◽  
Nbo Analysis ◽  
Vibrational Frequencies ◽  
Molecular Structures ◽  
Basis Set ◽  
Functional Theory ◽  
B3lyp Method

The optimized molecular structures and vibrational frequencies and also gauge including atomic orbital (GIAO)1H and13C NMR shift values of benzoylcholine chloride [(2-benzoyloxyethyl) trimethyl ammonium chloride] have been calculated using density functional theory (B3LYP) method with 6-31++G(d) basis set. The comparison of the experimental and calculated infrared (IR), Raman, and nuclear magnetic resonance (NMR) spectra has indicated that the experimental spectra are formed from the superposition of the spectra of two lowest energy conformers of the compound. So, it was concluded that the compound simultaneously exists in two optimized conformers in the ground state. Also the natural bond orbital (NBO) analysis has supported the simultaneous exiting of two conformers in the ground state. The calculated optimized geometric parameters (bond lengths and bond angles) and vibrational frequencies for both the lowest energy conformers were seen to be in a well agreement with the corresponding experimental data.

A Robust Non-Self-Consistent Tight-Binding Quantum Chemistry Method for large Molecules

2019 ◽  
Author(s):  
Philipp Pracht ◽  
Eike Caldeweyher ◽  
Sebastian Ehlert ◽  
Stefan Grimme
Keyword(s):  
Density Functional ◽  
Tight Binding ◽  
Computational Cost ◽  
Vibrational Frequencies ◽  
Molecular Structures ◽  
New Method ◽  
Basis Set ◽  
Semiempirical Methods ◽  
Electronic Interaction ◽  
Self Consistent

We propose a semiempirical quantum chemical method, designed for the fast calculation of molecular Geometries, vibrational Frequencies and Non-covalent interaction energies (GFN) of systems with up to a few thousand atoms. Like its predecessors GFN-xTB and GFN2-xTB, the new method termed GFN0-xTB is parameterized for all elements up to radon (Z = 86) and mostly shares well-known density functional tight-binding approximations as well as basis set and integral approximations. The main new feature is the avoidance of the self-consistent charge iterations leading to speed-ups of a factor of 2-20 depending on the size and electronic complexity of the system. This is achieved by including only quantum mechanical contributions up to first-order which are incorporated similar to the previous versions without any pair-specific parameterization. The essential electrostatic electronic interaction is treated by a classical electronegativity equilibration charge model yielding atomic partial charges that enter the electronic Hamiltonian indirectly. Furthermore, the atomic charge-dependent D4 dispersion correction is included to account for long range London correlation effects. Formulas for analytical total energy gradients with respect to nuclear displacements are derived and implemented in the <i>xtb </i>code allowing numerically very precise structure optimizations. The neglect of self-consistent energy terms not only leads to a large gain in computational speed but also can increase robustness in electronically difficult situations because ill-convergence or artificial charge-transfer (CT) is avoided. The comparison of GFN0-xTB and GFN/GFN2-xTB allows dissection of quantum electronic polarization and CT effects thereby improving our understanding of chemical bonding. Compared to the most sophisticated multipole-based GFN2-xTB model (which approaches DFT accuracy for the target properties closely), GFN0-xTB performs slightly worse for non-covalent interactions and molecular structures, while very good results are observed for conformational energies. Vibrational frequencies are obtained less accurately than with GFN/GFN2-xTB but they may still be useful for various purposes like estimating relative thermostatistical reaction energies. Most exceptional is the fact that even relatively complicated transition metal complex structures can be accurately optimized with a non-self-consistent quantum approach. The new method bridges the gap between force-fields and traditional semiempirical methods with its excellent computational cost to accuracy ratio and is intended to explore the chemical space of large molecular systems and solids.<br>

A Robust Non-Self-Consistent Tight-Binding Quantum Chemistry Method for large Molecules

2019 ◽  
Author(s):  
Philipp Pracht ◽  
Eike Caldeweyher ◽  
Sebastian Ehlert ◽  
Stefan Grimme
Keyword(s):  
Density Functional ◽  
Tight Binding ◽  
Computational Cost ◽  
Vibrational Frequencies ◽  
Molecular Structures ◽  
New Method ◽  
Basis Set ◽  
Semiempirical Methods ◽  
Electronic Interaction ◽  
Self Consistent

We propose a semiempirical quantum chemical method, designed for the fast calculation of molecular Geometries, vibrational Frequencies and Non-covalent interaction energies (GFN) of systems with up to a few thousand atoms. Like its predecessors GFN-xTB and GFN2-xTB, the new method termed GFN0-xTB is parameterized for all elements up to radon (Z = 86) and mostly shares well-known density functional tight-binding approximations as well as basis set and integral approximations. The main new feature is the avoidance of the self-consistent charge iterations leading to speed-ups of a factor of 2-20 depending on the size and electronic complexity of the system. This is achieved by including only quantum mechanical contributions up to first-order which are incorporated similar to the previous versions without any pair-specific parameterization. The essential electrostatic electronic interaction is treated by a classical electronegativity equilibration charge model yielding atomic partial charges that enter the electronic Hamiltonian indirectly. Furthermore, the atomic charge-dependent D4 dispersion correction is included to account for long range London correlation effects. Formulas for analytical total energy gradients with respect to nuclear displacements are derived and implemented in the <i>xtb </i>code allowing numerically very precise structure optimizations. The neglect of self-consistent energy terms not only leads to a large gain in computational speed but also can increase robustness in electronically difficult situations because ill-convergence or artificial charge-transfer (CT) is avoided. The comparison of GFN0-xTB and GFN/GFN2-xTB allows dissection of quantum electronic polarization and CT effects thereby improving our understanding of chemical bonding. Compared to the most sophisticated multipole-based GFN2-xTB model (which approaches DFT accuracy for the target properties closely), GFN0-xTB performs slightly worse for non-covalent interactions and molecular structures, while very good results are observed for conformational energies. Vibrational frequencies are obtained less accurately than with GFN/GFN2-xTB but they may still be useful for various purposes like estimating relative thermostatistical reaction energies. Most exceptional is the fact that even relatively complicated transition metal complex structures can be accurately optimized with a non-self-consistent quantum approach. The new method bridges the gap between force-fields and traditional semiempirical methods with its excellent computational cost to accuracy ratio and is intended to explore the chemical space of large molecular systems and solids.<br>

THEORETICAL VIBRATIONAL SPECTRA STUDIES: THE EFFECT OF RING SIZE ON THE CARBONYL VIBRATIONAL FREQUENCIES

2007 ◽  
Vol 06 (03) ◽  
pp. 459-476 ◽  
Author(s):  
H. HOOSHYAR ◽  
H. RAHEMI ◽  
K. A. DILMAGANI ◽  
S. F. TAYYARI
Keyword(s):  
Density Functional ◽  
Vibrational Frequencies ◽  
Molecular Structures ◽  
Basis Sets ◽  
Torsional Angle ◽  
Ring Size ◽  
Stable Conformation ◽  
Functional Theory ◽  
Out Of Plane ◽  
Planar Conformer

In this paper, molecular structures and vibrational frequencies of cycloketone, cyclopropanone, cyclobutanone, cyclopentanone, and cyclohexanone have been investigated by density functional theory (DFT) and the second order Møller and Plesset (MP2) levels of theory, using 6-311G, 6-311G**, 6-311++G, and 6-311++G** basis sets. The calculations predict a planar structure for cyclopropanone. The cyclobutanone ring is puckered and according to the calculations at B3LYP/6-311++G** and MP2/6-311++G** levels of theory, it deviates from planarity by 5.0° and 15.7°, respectively. The calculated barrier height between puckered and planar conformer, at MP2/6-311G** level of theory, is 196.7 cm-1. The puckering frequency at B3LYP/6-311G** and MP2/6-311G** levels are 47.5 cm-1 and 121.2 cm-1. Cyclopentanone has twisted conformations ( C 2 symmetry) in which carbons C1 and C2 are out-of-plane with a torsional angle of 11.5° (MP2/6-311G**) and the molecule goes from one conformation to the other via an envelope transition conformation ( C s symmetry). or planar hilltop conformation ( C 2v symmetry). The most stable conformation of the cyclohexanone is C s chair, which has no ring strain, is used as land mark.

Ab Initio Hartree-Fock and Density Functional Theory Study on Molecular Structures, Energies, and Vibrational Frequencies of 2-Amino-3-, 4-, and 5-Nitropyridine

2010 ◽  
Vol 65 (1-2) ◽  
pp. 107-112 ◽  
Author(s):  
Yusuf Sert ◽  
Fatih Ucun ◽  
Mustafa Böyükata
Keyword(s):  
Experimental Data ◽  
Density Functional Theory ◽  
Ab Initio ◽  
Density Functional ◽  
Vibrational Frequencies ◽  
Geometric Parameters ◽  
Molecular Structures ◽  
Basis Set ◽  
Functional Theory ◽  
Hartree Fock

AbstractThe molecular structures, vibrational frequencies, and corresponding vibrational assignments of 2-amino-3-, 4-, and 5-nitropyridine have been calculated by using ab initio Hartree-Fock (HF) and density functional theory (B3LYP) methods with 6-311++G(d,p) basis set level. The calculated vibrational frequencies and optimized geometric parameters (bond lengths and bond angles) were found to be in well agreement with the experimental data. The comparison of the observed and the calculated results showed that the scaled B3LYP method is superior to the scaled HF method for both the vibrational frequencies and the geometric parameters. For well fitting the calculated and the experimental frequencies we used scale factors obtained from the ratio of the frequency values of the strongest peaks in the calculated and the experimental spectra. These obtained scales seem to cause the better agreement of the gained vibrations to the experimental data.

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