1,721,312 research outputs found
With Computers from Atoms to Macromolecular Systems
No full textWe review and update selected contributions to computational chemistry made since the late 1950s. Introductory remarks are given to place our work in the context of contemporary science. We start with a classical benchmark, the H(2) wave-function constructed with a new one-particle representation, the Chemical spin-Orbitals, which replaces the traditional Atomic and Molecular spin-Orbitals. Computations from diatomic to small polyatomic molecules, obtained with the Hartree-Fock-Heitler-London (HF-HL) model, are compared to those obtained from the traditional Hartree-Fock (HF) and Heitler-London (HL) methods; we conclude that the hierarchy of solutions within the HF-HL approach represents a general and reasonable choice for computational quantum chemistry. Further, we show that a wave function constructed with Chemical spin-Orbitals is equivalent to a wave-function obtained with the HF-HL model. These simulations are complemented with a critical analysis on the correlation energy, and on Wigner and Coulomb Hole functionals.
The above studies follow the early Hartree-Fock period (1960-1970) characterised by pioneering computations on atomic and molecular systems, including basis set optimisation, atomic energy tabulation at the Hartree-Fock and post Hartree-Fock level, and potential energy surface computations obtained with the super-molecular approach. However, to deal with large molecular systems and to explicitly consider temperature and time, we must turn to statistical methods; we recall simulations using Monte Carlo, Molecular Dynamics, and Langevin dynamics, first at equilibrium, then for open systems at non-equilibrium. A concatenation of these models constitutes to the Global Simulation approach, discussed in detail.
The above work requires both computer hardware and application codes in different areas of computational chemistry. We recall the quantum chemical atomic and molecular codes and the statistical mechanics codes written, documented and freely distributed for the last half century. Further, we recall our pioneering efforts in the early 1980s in computer architecture, with the design and assembly of a parallel supercomputer extensively used to perform the first parallel applications in computational chemistry
Energy and Density Analysis on the H2 Molecule from the United Atom to Dissociationi: the Sigma, Pi, Delta, Phi and Gamma States.
No full textWe present 140 accurate potential energy curves, PECs, for the Sigma, Pi, Delta, Phi, and Gamma manifolds for the H(2) molecule, mapping all the states with energy below the H(2)(+) ground state. The full configuration interaction, nonrelativistic Born-Oppenheimer computations are performed with large and optimized basis sets of Slater-type and spherical Gaussian functions; these new basis sets are somewhat larger than those used in recent published studies on the 60 Sigma state PECs. The full CI computations are performed twice, with Hartree-Fock and with Heitler-London-type functions, allowing the identification of the ionic component in the total energy. The computed energies are within 10 (5) hartree from the most accurate PECs in literature. We aim ( a) at the evaluation of the PECs starting at very short and unexplored internuclear distances (0.01 bohrs) and ending at full dissociation, (b) at the systematic prediction of high excited state PECs dissociating as 1s + 4l and 1s + 5l, and ( c) at the characterization of the evolution of the 140 PEC electronic densities from united atom to dissociation. With this work we fill a gap in today literature, which has dealt mainly with low excited states, generally excluding short internuclear distances. The electronic configuration at the united atom persists as dominant configuration well beyond the equilibrium separation, and it switches to that at dissociation often with energy patterns seemingly irregular, in particular when the values of the principal quantum number at dissociation and at the united atom differ by one or more unit. The Hund's singlet-triplet splitting, which propagates from the united atom to the molecule, is discussed. The singlet and triplet states are rather close in energy in the Pi manifolds, and approach degeneracy in the Delta and Phi manifolds, to become fully degenerate in the Gamma manifolds. Discussions on the correlation energy correction, adiabatic correction, spectroscopic constants and on general features of the H(2) excited states are presented. The H(2) molecule is a system, which-to be understood-needs consideration of both the very short internuclear distances in approaching the united atom and of the very high excited states below H(2)(+). (C) 2010 Wiley Periodicals, Inc. Int J Quantum Chem 111: 3517-3540, 201
omments on computational chemistry: From diatomic molecules to large biochemical systems
No full textComments are presented on a few aspects of computational chemistry, considering its evolution and eventual future developments. We also discuss in some detail two topics: first, a new quantum chemical approach that proposes the combination of Hartree-Fock and Heitler-Lonclon approximations at the ab initio and at the semi-empirical level and, second, a sketch for a computational approach aimed at simulations of biological systems with cellular complexity, following an introductory excursion through Newton and Langevin molecular dynamics, microdynamics, and cellular automata
Monte Carlo simulation of the interaction between water solvent and biomolecules: glycine and the corresponding zwitterion
No full textStudy of the electronic structure of molecules. XXIII. Decomposition of correlation energy into atomic, covalent, ionic and van der Waals components
No full textMonte Carlo simulation of water solvent with biomolecules: glycine and the corresponding zwitterion
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