Search results for "Computation"

17O NMR study of chlorinated anisoles

1988

The 17O NMR chemical shifts of anisole and all 19 chlorinated anisoles measured in CDCI3 at ambient temperature show a dispersion of approximately 40 ppm. The substituent chemical shifts (SCS) of di- or more highly chlorinated compounds cannot be reproduced by addition of single substituent effects. Theoretical calculations of the 17O NMR chemical shifts have been attempted by molecular mechanical and semi-empirical molecular orbital methods at the INDO level, but the results are not satisfactory.

DFT calculations of structures, 13C NMR chemical shifts, and Raman RBM mode of simple models of small-diameter zigzag (4,0) carboxylated single-walle…

2012

Linearly conjugated benzene rings (acenes), belt-shaped molecules (cyclic acenes), and models of single-walled carbon nanotubes (SWCNTs) with one carboxylic group at the open end were fully optimized at the B3LYP/6-31G* level of theory. These models were selected to obtain some insight into the nuclear isotropic changes resulting from systematically increasing the basic building units of open-tip-monocarboxylated SWCNTs. In addition, the position of radial breathing mode (RBM), empirically correlated with the SWCNT diameter, was directly related with the radius of model cyclic acene rings. A regular convergence of selected structural, NMR, and Raman parameters with the molecular system size…

Interacting induced dipoles polarization model for molecular polarizabilities. Reference molecules, amino acids and model peptides

1999

Abstract We outline a method for the calculation of molecular dipole ( μ ) and quadrupole ( Θ = ) moments and dipole–dipole polarizabilities ( α = ) which we have successfully applied to a series of reference molecules, amino acids and model peptides. The results for μ are in line with CPHF reference calculations. In particular, the calculated positive value of CO is in agreement with both experimental and CI calculations. The computation of ( α = ) has been performed by the interacting induced dipoles polarization model that calculates tensor effective anisotropic point polarizabilities (method of Applequist et al.). The POLAR program cannot be used as a black box. Some tests should be per…

Analytic calculation of the diagonal Born-Oppenheimer correction within configuration-interaction and coupled-cluster theory

2006

Schemes for the analytic calculation of the diagonal Born-Oppenheimer correction (DBOC) are formulated and implemented for use with general single-reference configuration-interaction and coupled-cluster wave function models. Calculations are reported to demonstrate the convergence of the DBOC with respect to electron-correlation treatment and basis set as well as to investigate the size-consistency error in configuration-interaction calculations of the DBOC. The importance of electron-correlation contributions to the DBOC is illustrated in the computation of the corresponding corrections for the reaction energy and activation barrier of the F + H2 --FH + H reaction as well as of the atomiza…

Analytic first derivatives for a spin-adapted open-shell coupled cluster theory: Evaluation of first-order electrical properties

2014

An analytic scheme is presented for the evaluation of first derivatives of the energy for a unitary group based spin-adapted coupled cluster (CC) theory, namely, the combinatoric open-shell CC (COSCC) approach within the singles and doubles approximation. The widely used Lagrange multiplier approach is employed for the derivation of an analytical expression for the first derivative of the energy, which in combination with the well-established density-matrix formulation, is used for the computation of first-order electrical properties. Derivations of the spin-adapted lambda equations for determining the Lagrange multipliers and the expressions for the spin-free effective density matrices for…

Characterization of Polypeptides and Polypeptoides - Methods and Challenges

2017

A MRCI PS and CASSCF study of the ground state MgO dissociation energy

1991

Abstract Ab initio calculations at CASSCF and MRCI PS levels are used to determine the dissociation energy for the X 1 Σ + state of MgO, which adiabatically dissociates to the ground state 1 S g of magnesium and to the excited 1 D g state of oxygen, as well as other spectroscopic parameters. Emphasis is placed upon the problem of properly selecting an adequate active space in CASSCF calculations and upon the improvements obtained in MRCI by selecting perturbatively the most important contributions to the total wavefunction and evaluating the remaining ones only by perturbational method. Through a procedure based on stabilizing the computed dissociation energy, values of 3.87 eV (MRCI PS) an…

Structural and Spectroscopic Properties of Mg−Bacteriochlorin and Methyl Bacteriochlorophyllidesa,b,g, andhStudied by Semiempirical, ab Initio, and D…

2004

Ab initio HF/6-31G* and density functional B3LYP/6-31G* methods have been used to calculate fully optimized structures of methyl bacteriochlorophyllides a, b, g, and h and magnesium-bacteriochlorin...

Molecular Material‐Based Conductimetric Gas Sensors

2012

A mutliconfigurational study of low-lying electronic states of KO

1992

Abstract Potential energy curves and spectroscopic parameters of several electronic states of the KO molecule have been calculated using multi-configurational methods. The KO B 2Π state, first time theoretically described, presents a strong avoided crossing with the A 2Π state, and allows for the explanation of the observed fluorescence of the KO molecule. Eleven electronic states have been studied at all the internuclear distances. Effects of complete active space and basis set selections on the results are also analyzed.