1,721,235 research outputs found
A new formulation of the PCM solvation method: PCM-QINTn RID E-4986-2010
No full textA new formulation of the PCM electrostatic solution problem is proposed. Through a new derivation of the PCM-CLSn expression we propose an interpolation formula that improved the convergence: PCM-QINTn. All the available formulations are applied to the evaluation of the electrostatic component of the free energy of solvation for some molecular systems. In addition, PCM-QINT derivatives of G(el) with respect to atomic coordinates are evaluated. The computational costs are compared with those of PCM-direct formulation
Ab initio study of the S(N)2 reaction CH3Cl+Cl-->Cl-+CH3Cl in supercritical water with the polarizable continuum model RID E-4986-2010
No full textWe present some calculations for an S(N)2 reaction in ambient and supercritical water performed with a continuum method for solvation, the polarizable continuum model (PCM). PCM allows us to perform quantum mechanical (QM) ab initio calculations at almost all the levels of the QM theory and allows us to treat solvent-separated solutes on the same footing as a tight reacting system. Both features are here exploited. The results indicate that the standard PCM version is able to reproduce computer simulation results at a good level of accuracy, with the exception of a specific range of P-T values of supercritical water characterized by a large isothermal compressibility. For calculations in this P-T range we present a PCM formulation giving results in good agreement with results from simulations
DefPol: New procedure to build molecular surfaces and its use in continuum solvation methods RID E-4986-2010
No full textWe present a method to define van der Waals, solvent-accessible, and solvent-excluding molecular surfaces with their partition in nonoverlapping surface portions (tesserae). The procedure is more efficient than those available in the literature to describe solvent effects on molecular systems of large size, and it tan also be applied to solutes of small size without reducing the accuracy of the output and without increasing computational times. All the tesserae are expressed in terms of spherical triangles, having all the characterizing elements (vertices, centers, etc.) analytically defined. The method was tested by comparing the results for the surface area and the solvation free energy (decomposed in electrostatic, dispersion, and steric contributions) obtained using the GEPOL procedure within the framework of the polarizable continuum model solvation method. These comparisons regard 87 molecules at the molecular mechanics level and 28 molecules at the ab initio Hartree-Fock level: the results art? quite satisfactory. (C) 1998 John Wiley & Sons, Inc
Medium Effects on the Properties of Chemical Systems: an Overview of Recent Reformulations in the Polarizable Continuum Model (PCM)
No full textA summary of the outstanding characteristics of the polarizable continuum model ŽPCM., compared with other effective Hamiltonian methods to treat chemical problems in solution, is accompanied with a more detailed exposition of the computational features recently introduced for the calculation of molecular properties in solution. These features regard the analytical calculation of first and second derivatives of the free energy with respect to nuclear coordinates Žwith the first complete code for second derivatives., the HartreeFock and KohnSham formulations for coupled perturbed calculations, the vibrational and electronic hyperpolarizabilities Žboth static and frequency dependent., the local field corrections to hyperpolarizabilities, the nuclear magnetic shielding, and the vibrational circular dichroism. Numerical examples are reported in a companion study
Electronic excitation energies of molecules in solution within continuum solvation models: Investigating the discrepancy between state specific and linear response methods
No full textIn a recent article (R. Cammi, S. Corni, B. Mennucci, and J. Tomasi, J. Chem. Phys. 122, 104513,2005), we demonstrated that the state-specific (SS) and the linear-response (LR) approaches, two different ways to calculate solute excitation energies in the framework of quantum-mechanical continuum models of solvation, give different excitation energy expressions. In particular, they differ in the terms related to the electronic response of the solvent. In the present work, we further investigate this difference by comparing the excitation energy expressions of SS and LR with those obtained through a simple model for solute-solvent systems that bypasses one of the basic assumptions of continuum solvation models, i.e., the use of a single Hartree product of a solute and a solvent wave function to describe the total solute-solvent wave function. In particular, we consider the total solute-solvent wave function as a linear combination of the four products of two solute states and two solvent electronic states. To maximize the comparability with quantum-mechanical continuum model the resulting excitation energy expression is recast in terms of response functions of the solvent and quantities proper for the solvated molecule. The comparison of the presented expressions with the LR and SS ones enlightens the physical meaning of the terms included or neglected by these approaches and shows that SS agrees with the results of the four-level model, while LR includes a term classified as dispersion in previous treatments and neglects another related to electrostatic. A discussion on the possible origin of the LR flaw is finally given
Solvation energy with the ab initio PCM-QINT method: tautomeric equilibria
No full textWe present here a new version of the polarizable continuum model (PCM) for the ab-initio description of solvent effects on molecular solutes. This new version gives the free energy of the system and the wave function of the solute in terms of an approximated formulation that can be selected within a series of approximations exhibiting a rapid convergence. We present an application of the method to some tautomeric equilibria: the error with respect to the accurate value is less than 0.1 kcal mol(-1) if the approximation corresponds to the fourth term in the series, and less than 0.25 kcal mol(-1) when the approximation correspond to the second term in the series. Thanks to this new version of PCR;I geometry optimizations become faster and the amount of intermediate data to be stored during the calculations is reduced. (C) 1998 Elsevier Science B.V. All rights reserved
Analytical derivatives for molecular solutes .3. Hartree-Fock static polarizability and hyperpolarizabilities in the polarizable continuum model
No full textASPECTS OF ELECTROPHILIC BROMINATION OF ALKENES IN SOLUTION - THEORETICAL CALCULATION OF ATOMIC CHARGES IN BROMONIUM IONS
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