Computational chemistry is a branch of chemistry that uses computer simulation to assist in solving chemical problems. It uses methods of theoretical chemistry,incorporated into computer programs,to calculate the structures and properties of molecules,groups of molecules,and solids. It is essential because,apart from relatively recent results concerning the hydrogen molecular ion,the quantum many-body problem cannot be solved analytically,much less in closed form. While computational results normally complement the information obtained by chemical experiments,it can in some cases predict hitherto unobserved chemical phenomena. It is widely used in the design of new drugs and materials.
Renormalization is a collection of techniques in quantum field theory,the statistical mechanics of fields,and the theory of self-similar geometric structures,that are used to treat infinities arising in calculated quantities by altering values of these quantities to compensate for effects of their self-interactions. But even if no infinities arose in loop diagrams in quantum field theory,it could be shown that it would be necessary to renormalize the mass and fields appearing in the original Lagrangian.
The density matrix renormalization group (DMRG) is a numerical variational technique devised to obtain the low-energy physics of quantum many-body systems with high accuracy. As a variational method,DMRG is an efficient algorithm that attempts to find the lowest-energy matrix product state wavefunction of a Hamiltonian. It was invented in 1992 by Steven R. White and it is nowadays the most efficient method for 1-dimensional systems.
In physics,an effective field theory is a type of approximation,or effective theory,for an underlying physical theory,such as a quantum field theory or a statistical mechanics model. An effective field theory includes the appropriate degrees of freedom to describe physical phenomena occurring at a chosen length scale or energy scale,while ignoring substructure and degrees of freedom at shorter distances. Intuitively,one averages over the behavior of the underlying theory at shorter length scales to derive what is hoped to be a simplified model at longer length scales. Effective field theories typically work best when there is a large separation between length scale of interest and the length scale of the underlying dynamics. Effective field theories have found use in particle physics,statistical mechanics,condensed matter physics,general relativity,and hydrodynamics. They simplify calculations,and allow treatment of dissipation and radiation effects.
Carbon-12 (12C) is the most abundant of the two stable isotopes of carbon,amounting to 98.93% of element carbon on Earth;its abundance is due to the triple-alpha process by which it is created in stars. Carbon-12 is of particular importance in its use as the standard from which atomic masses of all nuclides are measured,thus,its atomic mass is exactly 12 daltons by definition. Carbon-12 is composed of 6 protons,6 neutrons,and 6 electrons.
Lattice QCD is a well-established non-perturbative approach to solving the quantum chromodynamics (QCD) theory of quarks and gluons. It is a lattice gauge theory formulated on a grid or lattice of points in space and time. When the size of the lattice is taken infinitely large and its sites infinitesimally close to each other,the continuum QCD is recovered.
The Vienna Ab initio Simulation Package,better known as VASP,is a package for performing ab initio quantum mechanical calculations using either Vanderbilt pseudopotentials,or the projector augmented wave method,and a plane wave basis set. The basic methodology is density functional theory (DFT),but the code also allows use of post-DFT corrections such as hybrid functionals mixing DFT and Hartree–Fock exchange,many-body perturbation theory and dynamical electronic correlations within the random phase approximation (RPA) and MP2.
A three-body force is a force that does not exist in a system of two objects but appears in a three-body system. In general,if the behaviour of a system of more than two objects cannot be described by the two-body interactions between all possible pairs,as a first approximation,the deviation is mainly due to a three-body force.
PQS is a general purpose quantum chemistry program. Its roots go back to the first ab initio gradient program developed in Professor Peter Pulay's group but now it is developed and distributed commercially by Parallel Quantum Solutions. There is a reduction in cost for academic users and a site license. Its strong points are geometry optimization,NMR chemical shift calculations,and large MP2 calculations,and high parallel efficiency on computing clusters. It includes many other capabilities including Density functional theory,the semiempirical methods,MINDO/3,MNDO,AM1 and PM3,Molecular mechanics using the SYBYL 5.0 Force Field,the quantum mechanics/molecular mechanics mixed method using the ONIOM method,natural bond orbital (NBO) analysis and COSMO solvation models. Recently,a highly efficient parallel CCSD(T) code for closed shell systems has been developed. This code includes many other post Hartree–Fock methods:MP2,MP3,MP4,CISD,CEPA,QCISD and so on.
Jaguar is a computer software package used for ab initio quantum chemistry calculations for both gas and solution phases. It is commercial software marketed by the company Schrödinger. The program was originated in research groups of Richard Friesner and William Goddard and was initially called PS-GVB.
The Lippmann–Schwinger equation is one of the most used equations to describe particle collisions –or,more precisely,scattering –in quantum mechanics. It may be used in scattering of molecules,atoms,neutrons,photons or any other particles and is important mainly in atomic,molecular,and optical physics,nuclear physics and particle physics,but also for seismic scattering problems in geophysics. It relates the scattered wave function with the interaction that produces the scattering and therefore allows calculation of the relevant experimental parameters.
Car–Parrinello molecular dynamics or CPMD refers to either a method used in molecular dynamics or the computational chemistry software package used to implement this method.
Beryllium-8 is a radionuclide with 4 neutrons and 4 protons. It is an unbound resonance and nominally an isotope of beryllium. It decays into two alpha particles with a half-life on the order of 8.19×10−17 seconds. This has important ramifications in stellar nucleosynthesis as it creates a bottleneck in the creation of heavier chemical elements. The properties of 8Be have also led to speculation on the fine tuning of the Universe,and theoretical investigations on cosmological evolution had 8Be been stable.
CP2K is a freely available (GPL) quantum chemistry and solid state physics program package,written in Fortran 2008,to perform atomistic simulations of solid state,liquid,molecular,periodic,material,crystal,and biological systems. It provides a general framework for different methods:density functional theory (DFT) using a mixed Gaussian and plane waves approach (GPW) via LDA,GGA,MP2,or RPA levels of theory,classical pair and many-body potentials,semi-empirical and tight-binding Hamiltonians,as well as Quantum Mechanics/Molecular Mechanics (QM/MM) hybrid schemes relying on the Gaussian Expansion of the Electrostatic Potential (GEEP). The Gaussian and Augmented Plane Waves method (GAPW) as an extension of the GPW method allows for all-electron calculations. CP2K can do simulations of molecular dynamics,metadynamics,Monte Carlo,Ehrenfest dynamics,vibrational analysis,core level spectroscopy,energy minimization,and transition state optimization using NEB or dimer method.
The index of physics articles is split into multiple pages due to its size.
The light-front quantization of quantum field theories provides a useful alternative to ordinary equal-time quantization. In particular,it can lead to a relativistic description of bound systems in terms of quantum-mechanical wave functions. The quantization is based on the choice of light-front coordinates,where plays the role of time and the corresponding spatial coordinate is . Here, is the ordinary time, is a Cartesian coordinate,and is the speed of light. The other two Cartesian coordinates, and ,are untouched and often called transverse or perpendicular,denoted by symbols of the type Failed to parse:{\vec x}_{\perp}=(x,y) . The choice of the frame of reference where the time and -axis are defined can be left unspecified in an exactly soluble relativistic theory,but in practical calculations some choices may be more suitable than others. The basic formalism is discussed elsewhere.
In nuclear physics,ab initio methods seek to describe the atomic nucleus from the bottom up by solving the non-relativistic Schrödinger equation for all constituent nucleons and the forces between them. This is done either exactly for very light nuclei or by employing certain well-controlled approximations for heavier nuclei. Ab initio methods constitute a more fundamental approach compared to e.g. the nuclear shell model. Recent progress has enabled ab initio treatment of heavier nuclei such as nickel.
James P. Vary is an American theoretical physicist and professor at the Iowa State University,specializing in nuclear theory with an emphasis on "ab initio" solutions of quantum many-particle systems and light-front quantum field theory.
G. Peter Lepage is a Canadian American theoretical physicist and an academic administrator. He was the Harold Tanner Dean of the College of Arts and Sciences at Cornell University from 2003 to 2013.
Zoltan Fodor is a Hungarian theoretical particle physicist,best known for his works in lattice QCD by numerically solving the theory of the strong interactions.
