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Краснощеков С.В., стажировка в Ulm University, Ulm, Germanyстажировка в организации
- Сотрудник: Краснощеков Сергей Вадимович
- Отправляющая организация: МГУ имени М.В. Ломоносова
- Принимающая организация: Ulm University, Ulm, Germany, Германия
- Отправляющее подразделение: Кафедра физической химии
- Направление: Стажировка сотрудника подразделения во внешней организации
- Дата начала: 1 февраля 2018 г.
- Дата окончания: 23 февраля 2018 г.
- Цель стажировки: The field of research conducted during my stay in Ulm is a precise determination of equilibrium molecular structures (with typical accuracy of the order of ~0.0002 Å) of a wide class of organic molecules on the basis of combining experimental data obtained from a series of microwave measurements combined with high level quantum mechanical calculations. The equilibrium geometry (re structure) is a key quantitative information about molecular structure. Ulm University maintains the database MOGADOC is putting together equilibrium structures of many studied molecules, including original studies in Ulm. Since molecules undergo “zero-state” vibrations with large amplitudes (up to ~0.05 Å), the rotational constants B0 obtained experimentally are related to vibrationally averaged molecular structures, which are different from equilibrium structures due to typically significant anharmonic curvature of a molecular potential function along normal coordinate modes of vibration. This effect can be accounted for by calculation of so-called rotation-vibrational interaction constants , which can be obtained by using cubic force field in normal coordinates and the second-order vibrational perturbation theory; they contain cubic force constants in dimensionless normal coordinates, . Rovibrational corrections to ground state rotational constants are composed of the vibration-rotation constants summed over all normal modes. The major problem is that calculation of such constants is a very time-consuming procedure, if standard quantum-mechanical software is employed (e.g., Gaussian™). For large molecules, full calculation for a series of isotopes using modern coupled clusters models can last for several months because a new calculation must be performed for each additional isotopologue. In this work, we have solved this problem. First, we have written a new software that reduces quantum-mechanical the calculation to only one isotopic species. Besides, we have developed a new method for transformation of normal coordinate force field to Cartesian coordinate representation, which can also reduce computational time [N.Vogt, J.Demaison, S.V.Krasnoshchekov et al. “Efficient transformations between normal coordinate anharmonic force fields of isotopologues: Semiexperimental equilibrium structure of proline”, Mol.Phys., 2017, 115:942]. For this transformation, we introduced a new matrix, , where Q are normal coordinates and X are Cartesian coordinates. One this matrix is known, the force field in Cartesian coordinates is given by: , (1) The b-matrix can be obtained from following expression, , (2) where the matrices in the right side are squared harmonic frequencies (λ), vibrational forms (L), internal coordinate force constants (F) and the definition of internal coordinates (B). The correctness of Eq. (1) was checked numerically by converting normal coordinate cubic force field of proline to Cartesian coordinate representations, and back to normal using Eq. (2). Both fields in normal coordinates, original and restored, were found identical to machine accuracy. It is very convenient to calculate the cubic force directly in normal coordinates since it is generally more accurate and less time consuming routine due to saving on translational and rotational degrees of freedom. During my work in Ulm, our original software package ANCO (written in Fortran) was upgraded and tuned to needs of the scientific group that is specialized to serial evaluation of precise molecular structures. We conducted a series of tests on the proline molecule (see Ref. above) and have proven that the new procedure yields correct rotation-vibrational interaction constants for arbitrary isotopologues after calculation of the Cartesian force field by Gaussian™ program for the parent molecule, conversion of the force field to one for dedicated isotopologues and a new calculation of the isotopic species with prepared data. Performed tests indicated the correctness of obtained quantities. The software manual was written as well. The program was transferred to local specialists and their training was performed. Performed tests on proline molecule have proven correctness of obtained results.