1% 2% $Id$ 3% 4\label{sec:functionality} 5 6NWChem provides many methods to compute the properties of molecular and 7periodic systems using standard quantum mechanical descriptions of the 8electronic wavefunction or density. In addition, NWChem has the 9capability to perform classical molecular dynamics and free energy 10simulations. These approaches may be combined to perform mixed 11quantum-mechanics and molecular-mechanics simulations. 12 13NWChem is available on almost all high performance computing platforms, 14workstations, PCs running LINUX, as well as clusters of desktop platforms or 15workgroup servers. NWChem development has been devoted to providing 16maximum efficiency on massively parallel processors. It achieves this performance 17on the 1960 processors HP Itanium2 system in the EMSL's MSCF. 18It has not been optimized for high performance on single processor desktop systems. 19 20\section{Molecular electronic structure} 21 22The following quantum mechanical methods are available to calculate 23energies, analytic first derivatives and second derivatives with respect to atomic 24coordinates. 25 26\begin{itemize} 27\item Self Consistent Field (SCF) or Hartree Fock (RHF, UHF). 28\item Gaussian Density Functional Theory (DFT), using many local, 29 non-local (gradient-corrected), and hybrid (local, non-local, and HF) 30exchange-correlation potentials 31(spin-restricted) 32with formal $N^3$ and $N^4$ scaling. 33\end{itemize} 34 35The following methods are available to calculate energies and analytic 36first derivatives with respect to atomic coordinates. Second derivatives 37are computed by finite difference of the first derivatives. 38 39\begin{itemize} 40\item Self Consistent Field (SCF) or Hartree Fock (ROHF). 41\item Gaussian Density Functional Theory (DFT), using many local, 42 non-local (gradient-corrected), and hybrid (local, non-local, and HF) 43exchange-correlation potentials 44(spin-unrestricted) 45with formal $N^3$ and $N^4$ scaling. 46\item Spin-orbit DFT (SODFT), using many local and non-local (gradient-corrected) 47exchange-correlation potentials (spin-unrestricted). 48\item MP2 including semi-direct using frozen core and RHF and UHF reference. 49\item Complete active space SCF (CASSCF). 50\end{itemize} 51 52The following methods are available to compute energies only. First 53and second derivatives are computed by finite difference of the 54energies. 55\begin{itemize} 56\item CCSD, CCSD(T), CCSD+T(CCSD), with RHF reference. 57\item Selected-CI with second-order perturbation correction. 58\item MP2 fully-direct with RHF reference. 59\item Resolution of the identity integral approximation MP2 (RI-MP2), with 60 RHF and UHF reference. 61\item CIS, TDHF, TDDFT, and Tamm--Dancoff TDDFT for excited states with RHF, UHF, RDFT, or UDFT reference. 62\item CCSD(T) and CCSD[T] for closed- and open-shell systems (TCE module) 63\item UCCD, ULCCD, UCCSD, ULCCSD, UQCISD, UCCSDT, and UCCSDTQ with RHF, UHF, or ROHF reference. 64\item UCISD, UCISDT, and UCISDTQ with RHF, UHF, or ROHF reference. 65\item Non-canonical UMP2, UMP3, and UMP4 with RHF or UHF reference. 66\item EOM-CCSD, EOM-CCSDT, EOM-CCSDTQ for excitation energies, transition 67moments, and excited-state dipole moments of closed- and open-shell 68systems 69\item CCSD, CCSDT, CCSDTQ for dipole moments of closed- and open-shell 70systems 71\end{itemize} 72 73For all methods, the following operations may be performed: 74\begin{itemize} 75\item Single point energy 76\item Geometry optimization (minimization and transition state) 77\item Molecular dynamics on the fully {\em ab initio} potential energy 78 surface 79\item Numerical first and second derivatives automatically computed if 80 analytic derivatives are not available 81\item Normal mode vibrational analysis in cartesian coordinates 82\item ONIOM hybrid method of Morokuma and co-workers 83\item Generation of the electron density file for graphical display 84\item Evaluation of static, one-electron properties. 85\item Electrostatic potential fit of atomic partial charges (CHELPG method with 86 optional RESP restraints or charge constraints) 87\end{itemize} 88 89For closed and open shell SCF and DFT: 90\begin{itemize} 91\item COSMO energies - the continuum solvation `COnductor-like Screening MOdel' 92 of A. Klamt and G. Sch\"{u}\"{u}rmann to describe dielectric screening effects in 93 solvents. 94\end{itemize} 95 96In addition, automatic interfaces are provided to 97\begin{itemize} 98%\item The COLUMBUS multi-reference CI package 99\item Python 100\item the POLYRATE direct dynamics software 101\end{itemize} 102 103\section{Relativistic effects} 104 105The following methods for including relativity in quantum chemistry 106calculations are available: 107\begin{itemize} 108\item Spin-free and spin-orbit one-electron Douglas-Kroll and zeroth-order 109regular approximations (ZORA) are available for all quantum mechanical 110methods and their gradients. 111\item Dyall's spin-free Modified Dirac Hamiltonian approximation is available 112 for the Hartree-Fock method and its gradients. 113\item One-electron spin-orbit effects can be included via spin-orbit potentials. 114 This option is available for DFT and its gradients, but has to be run without 115 symmetry. 116\end{itemize} 117 118\section{Pseudopotential plane-wave electronic structure} 119 120Two modules are available to compute the energy, optimize the 121geometry, numerical second derivatives, and perform ab initio 122molecular dynamics using pseudopotential plane-wave DFT. 123 124\begin{itemize} 125\item PSPW - (Pseudopotential plane-wave) A gamma point code for calculating 126molecules, liquids, crystals, and surfaces. 127\item Band - A prototype band structure code for calculating crystals and 128surfaces with small band gaps (e.g. semi-conductors and metals) 129\end{itemize} 130 131With 132 133\begin{itemize} 134\item Conjugate gradient and limited memory BFGS minimization 135\item Car-Parrinello (extended Lagrangian dynamics) 136\item Constant energy and constant temperature Car-Parrinello simulations 137\item Fixed atoms in cartesian and SHAKE constraints in Car-Parrinello 138\item Pseudopotential libraries 139\item Hamann and Troullier-Martins norm-conserving pseudopotentials with 140optional semicore corrections 141\item Automated wavefunction initial guess, now with LCAO 142\item Vosko and PBE96 exchange-correlation potentials (spin-restricted 143and unrestricted) 144\item Orthorhombic simulation cells with periodic and 145free space boundary conditions. 146\item Modules to convert between small and large plane-wave expansions 147\item Interface to DRIVER, STEPPER, and VIB modules 148\item Polarization through the use of point charges 149\item Mulliken, point charge, DPLOT (wavefunction, density and electrostatic 150potential plotting) analysis 151\end{itemize} 152 153%\section{Periodic system electronic structure} 154%A module (Gaussian Approach to Polymers, Surfaces and Solids (GAPSS)) 155%is available to compute energies by Gaussian Density 156%Functional Theory (DFT) with many local and non-local 157%exchange-correlation potentials. 158 159\section{Molecular dynamics} 160 161The following functionality is available for classical molecular 162simulations: 163\begin{itemize} 164\item Single configuration energy evaluation 165\item Energy minimization 166\item Molecular dynamics simulation 167\item Free energy simulation (multistep thermodynamic perturbation (MSTP) or 168 multiconfiguration thermodynamic integration (MCTI) methods with 169 options of single and/or dual topologies, double wide sampling, and 170 separation-shifted scaling) 171\end{itemize} 172 173The classical force field includes: 174\begin{itemize} 175\item Effective pair potentials (functional form used in AMBER, GROMOS, 176 CHARMM, etc.) 177\item First order polarization 178\item Self consistent polarization 179\item Smooth particle mesh Ewald (SPME) 180\item Twin range energy and force evaluation 181\item Periodic boundary conditions 182\item SHAKE constraints 183\item Consistent temperature and/or pressure ensembles 184\end{itemize} 185 186NWChem also has the capability to combine classical and quantum 187descriptions in order to perform: 188\begin{itemize} 189\item Mixed quantum-mechanics and molecular-mechanics (QM/MM) 190 minimizations and molecular dynamics simulation , and 191\item Quantum molecular dynamics simulation by using any of the quantum 192 mechanical methods capable of returning gradients. 193\end{itemize} 194 195By using the DIRDYVTST module of NWChem, the user can write an input 196file to the POLYRATE program, which can be used to calculate rate 197constants including quantum mechanical vibrational energies and tunneling 198contributions. 199 200\section{Python} 201 202The Python programming language has been embedded within NWChem and 203many of the high level capabilities of NWChem can be easily combined 204and controlled by the user to perform complex operations. 205 206\section{Parallel tools and libraries (ParSoft)} 207 208\begin{itemize} 209\item Global arrays (GA) 210\item Agregate Remote Memory Copy Interface (ARMCI) 211\item Linear Algebra (PeIGS) and FFT 212\item ParIO 213\item Memory allocation (MA) 214\end{itemize} 215 216