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\label{sec:functionality}

NWChem provides many methods to compute the properties of molecular and
periodic systems using standard quantum mechanical descriptions of the
electronic wavefunction or density.  In addition, NWChem has the
capability to perform classical molecular dynamics and free energy
simulations.  These approaches may be combined to perform mixed
quantum-mechanics and molecular-mechanics simulations. 

NWChem is available on almost all high performance computing platforms,
workstations, PCs running LINUX, as well as clusters of desktop platforms or
workgroup servers. NWChem development has been devoted to providing
maximum efficiency on massively parallel processors. It achieves this performance
on the 1960 processors HP Itanium2 system in the EMSL's MSCF.  
It has not been optimized for high performance on single processor desktop systems.

\section{Molecular electronic structure}

The following quantum mechanical methods are available to calculate
energies, analytic first derivatives and second derivatives with respect to atomic
coordinates.  

\begin{itemize}
\item Self Consistent Field (SCF) or Hartree Fock (RHF, UHF).
\item Gaussian Density Functional Theory (DFT), using many local,
  non-local (gradient-corrected), and hybrid (local, non-local, and HF)
exchange-correlation potentials 
(spin-restricted)
with formal $N^3$ and $N^4$ scaling.
\end{itemize}

The following methods are available to calculate energies and analytic
first derivatives with respect to atomic coordinates.  Second derivatives 
are computed by finite difference of the first derivatives.

\begin{itemize}
\item Self Consistent Field (SCF) or Hartree Fock (ROHF).  
\item Gaussian Density Functional Theory (DFT), using many local,
  non-local (gradient-corrected), and hybrid (local, non-local, and HF)
exchange-correlation potentials 
(spin-unrestricted)
with formal $N^3$ and $N^4$ scaling.
\item Spin-orbit DFT (SODFT), using many local and non-local (gradient-corrected)
exchange-correlation potentials (spin-unrestricted).
\item MP2 including semi-direct using frozen core and RHF and UHF reference.
\item Complete active space SCF (CASSCF).
\end{itemize}

The following methods are available to compute energies only.  First
and second derivatives are computed by finite difference of the
energies.
\begin{itemize}
\item CCSD, CCSD(T), CCSD+T(CCSD), with RHF reference.
\item Selected-CI with second-order perturbation correction.
\item MP2 fully-direct with RHF reference.
\item Resolution of the identity integral approximation MP2 (RI-MP2), with
  RHF and UHF reference.
\item CIS, TDHF, TDDFT, and Tamm--Dancoff TDDFT for excited states with RHF, UHF, RDFT, or UDFT reference.
\item CCSD(T) and CCSD[T] for closed- and open-shell systems (TCE module)
\item UCCD, ULCCD, UCCSD, ULCCSD, UQCISD, UCCSDT, and UCCSDTQ with RHF, UHF, or ROHF reference.
\item UCISD, UCISDT, and UCISDTQ with RHF, UHF, or ROHF reference.
\item Non-canonical UMP2, UMP3, and UMP4 with RHF or UHF reference.
\item EOM-CCSD, EOM-CCSDT, EOM-CCSDTQ for excitation energies, transition
moments, and excited-state dipole moments of closed- and open-shell
systems
\item CCSD, CCSDT, CCSDTQ for dipole moments of closed- and open-shell
systems
\end{itemize}

For all methods, the following operations may be performed:
\begin{itemize}
\item Single point energy
\item Geometry optimization (minimization and transition state)
\item Molecular dynamics on the fully {\em ab initio} potential energy
  surface
\item Numerical first and second derivatives automatically computed if
  analytic derivatives are not available
\item Normal mode vibrational analysis in cartesian coordinates
\item ONIOM hybrid method of Morokuma and co-workers
\item Generation of the electron density file for graphical display
\item Evaluation of static, one-electron properties.
\item Electrostatic potential fit of atomic partial charges (CHELPG method with
    optional RESP restraints or charge constraints)
\end{itemize}

For closed and open shell SCF and DFT:
\begin{itemize}
\item COSMO energies - the continuum solvation `COnductor-like Screening MOdel'
    of A. Klamt and G. Sch\"{u}\"{u}rmann to describe dielectric screening effects in
    solvents.
\end{itemize}

In addition, automatic interfaces are provided to
\begin{itemize}
%\item The COLUMBUS multi-reference CI package
\item Python
\item the POLYRATE direct dynamics software
\end{itemize}

\section{Relativistic effects}

The following methods for including relativity in quantum chemistry 
calculations are available:
\begin{itemize}
\item Spin-free and spin-orbit one-electron Douglas-Kroll and zeroth-order
regular approximations (ZORA) are available for all quantum mechanical 
methods and their gradients.
\item Dyall's spin-free Modified Dirac Hamiltonian approximation is available 
 for the Hartree-Fock method and its gradients.
\item One-electron spin-orbit effects can be included via spin-orbit potentials.
 This option is available for DFT and its gradients, but has to be run without 
 symmetry.
\end{itemize}

\section{Pseudopotential plane-wave electronic structure}

Two modules are available to compute the energy, optimize the
geometry, numerical second derivatives, and perform ab initio 
molecular dynamics using pseudopotential plane-wave DFT.

\begin{itemize}
\item PSPW - (Pseudopotential plane-wave) A gamma point code for calculating
molecules, liquids, crystals, and surfaces.
\item Band - A prototype band structure code for calculating crystals and 
surfaces with small band gaps (e.g. semi-conductors and metals)
\end{itemize}

With

\begin{itemize}
\item Conjugate gradient and limited memory BFGS minimization
\item Car-Parrinello (extended Lagrangian dynamics)
\item Constant energy and constant temperature Car-Parrinello simulations
\item Fixed atoms in cartesian and SHAKE constraints in Car-Parrinello
\item Pseudopotential libraries
\item Hamann and Troullier-Martins norm-conserving pseudopotentials with 
optional semicore corrections
\item Automated wavefunction initial guess, now with LCAO
\item Vosko and PBE96 exchange-correlation potentials (spin-restricted 
and unrestricted)
\item Orthorhombic simulation cells with periodic and
free space boundary conditions.
\item Modules to convert between small and large plane-wave expansions
\item Interface to DRIVER, STEPPER, and VIB modules
\item Polarization through the use of point charges
\item Mulliken, point charge, DPLOT (wavefunction, density and electrostatic
potential plotting) analysis
\end{itemize}

%\section{Periodic system electronic structure}
%A module  (Gaussian Approach to Polymers, Surfaces and Solids (GAPSS))
%is available to compute energies by Gaussian Density
%Functional Theory (DFT) with many local and non-local
%exchange-correlation potentials.

\section{Molecular dynamics}

The following functionality is available for classical molecular
simulations:
\begin{itemize}
\item Single configuration energy evaluation
\item Energy minimization
\item Molecular dynamics simulation
\item Free energy simulation  (multistep thermodynamic perturbation (MSTP) or
    multiconfiguration thermodynamic integration (MCTI) methods with
    options of single and/or dual topologies, double wide sampling, and
    separation-shifted scaling)
\end{itemize}

The classical force field includes:
\begin{itemize}
\item Effective pair potentials (functional form used in AMBER, GROMOS,
    CHARMM, etc.) 
\item First order polarization
\item Self consistent polarization
\item Smooth particle mesh Ewald (SPME) 
\item Twin range energy and force evaluation 
\item Periodic boundary conditions
\item SHAKE constraints 
\item Consistent temperature and/or pressure ensembles
\end{itemize}

NWChem also has the capability to combine classical and quantum
descriptions in order to perform:
\begin{itemize}
\item Mixed quantum-mechanics and molecular-mechanics (QM/MM)
  minimizations and molecular dynamics simulation , and
\item Quantum molecular dynamics simulation by using any of the quantum
    mechanical methods capable of returning gradients.
\end{itemize}

By using the DIRDYVTST module of NWChem, the user can write an input
file to the POLYRATE program, which can be used to calculate rate
constants including quantum mechanical vibrational energies and tunneling
contributions.

\section{Python}

The Python programming language has been embedded within NWChem and
many of the high level capabilities of NWChem can be easily combined
and controlled by the user to perform complex operations.

\section{Parallel tools and libraries (ParSoft)}

\begin{itemize}
\item Global arrays (GA)
\item Agregate Remote Memory Copy Interface (ARMCI)
\item Linear Algebra (PeIGS) and FFT
\item ParIO
\item Memory allocation (MA)
\end{itemize}