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The size- and structure-dependence of the magnetic moment in Pd n clusters ( n = 13, 19, 38) was studied with density functional theory. We compare the Becke-Perdew gradient-corrected exchange-correlation functional with the parameterized meta-generalized gradient approximation functional by Zhao and Truhlar (M06-L). Although the former functional shows the expected quenching of the clusters' magnetic moment with increasing nuclearity, the latter one predicts a strongly increasing magnetic moment. In a series of fixed-spin calculations with the two functionals at different geometries, we observe a strong dependence of the magnetic moment on the cluster structure, where the untypically long metal-metal bonds predicted by the M06-L functional are a dominant influence on the stability of the high-spin cluster configurations. Furthermore, we observe differing behavior of the two functionals with respect to the magnetic moment as particular bonds in the cluster are strained, indicating that, in addition to the structural contribution, the description of exchange-correlation energy in the M06-L functional may intrinsically favor high magnetic moments in these Pd clusters. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012 [ABSTRACT FROM AUTHOR]

The PUPIL system is a combination of software and protocols for the systematic linkage and interoperation of molecular dynamics and quantum mechanics codes to perform QM/MD (sometimes called QM/MM) calculations. The Gaussian03 and Amber packages were added to the PUPIL suite recently. However, efficient parallel QM codes are critical because calculation of the QM forces is the overwhelming majority of the computational load. Here we report details of incorporation of the deMon2k density functional suite as a new parallel QM code. An additional motivation is to add a highly optimized, purely DFT code. We illustrate with a demonstration study of the influence of perchlorate as a dopant ion of the poly(3,4-ethylenedioxythiophene) conducting polymer in explicit acetonitrile solvent using Amber and deMon2k. We discuss unanticipated requirements for use of a scheme for semi-empirical correction of Kohn-Sham eigenvalues to give physically meaningful one-electron gap energies. We provide comparison of both geometric parameters and electronic properties for nondoped and doped systems. We also present results comparing deMon2k and Gaussian03 calculation of forces for a short sequence of steps. We discuss briefly some difficult problems of quantum zone SCF convergence for the anionically doped system. The difficulties seem to be caused by well-know deficiencies in simple approximate exchange-correlation functionals. © 2010 Wiley Periodicals, Inc. J Comput Chem, 2010 [ABSTRACT FROM AUTHOR]

Effective, explicitly density-dependent (i.e., orbital-free) approximations to the Kohn–Sham kinetic energy functional Ts remain elusive. In the course of developing such functionals in collaboration with Frank Harris, various issues have arisen that have gone unaddressed. We consider four of them here: positivity of the KS kinetic energy density, supposed requirements on its functional dependence, the use of cutoffs to insure positivity, and the role of the Coulomb virial theorem in the asymptotic analysis of Ts. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009 [ABSTRACT FROM AUTHOR]

The PUPIL package (Program for User Package Interfacing and Linking) originally was developed to interface different programs for multiscale calculations in materials sciences (Torras et al., J Comput Aided Mater Des 2006, 13, 201; Torras et al., Comput Phys Commun 2007, 177, 265). Here we present an extension of PUPIL to computational chemistry by interfacing two widely used computational chemistry programs: AMBER (molecular dynamics) and Gaussian (quantum chemistry). The benefit is to allow the application of the advanced MD techniques available in AMBER to a hybrid QM/MM system in which the forces and energy on the QM part can be computed by any of the methods available in Gaussian. To illustrate, we present two example applications: A MD calculation of alanine dipeptide in explicit water, and a use of the steered MD capabilities in AMBER to calculate the free energy of reaction for the dissociation of Angeli's salt. © 2008 Wiley Periodicals, Inc. J Comput Chem, 2008 [ABSTRACT FROM AUTHOR]

We derive and test an approximate expression for the Kohn–Sham eigenfunctions and total energy associated with a given exchange‐correlation functional. The expression is closely related to the approximation given by Benoit et al. (Phys Rev Lett 2001, 87, 226401) but differs by removing an overly stringent orthogonality constraint. Tests on the Li, F, Ne, and Cr atoms done in Gaussian basis sets suggest that the approach is promising, particularly as an alternative to conventional self‐consistent field or to the Harris scheme (Phys Rev B 1985, 31, 1770). © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2004 [ABSTRACT FROM AUTHOR]

Currently, the most common approach to calculate transport properties for materials under extreme conditions is based on the phenomenological Kubo‐Greenwood method. The results of an inquiry into the justification and context of that model are summarized here. Specifically, the basis for its connection to equilibrium density functional theory (DFT) and the assumption of static ions are discussed briefly. [ABSTRACT FROM AUTHOR]