Gaussian has been designed with the needs of the
user in mind. All of the standard input is free-format and mnemonic. Reasonable
defaults for input data have been provided, and the output is intended to be self-explanatory.
Mechanisms are available for the sophisticated user to override defaults or interface
their own code to the Gaussian system. The authors hope that their efforts
will allow users to concentrate their energies on the application of the methods
to chemical problems and to the development of new methods, rather than on the
mechanics of performing the calculations.
The technical capabilities of
the Gaussian 03 system are listed in the subsections below.
Calculation of one- and two-electron integrals over
any general contracted gaussian functions. The basis functions can either be cartesian
gaussians or pure angular momentum functions, and a variety of basis sets are
stored in the program and can be requested by name. Integrals may be stored in
memory, stored externally, or be recomputed as needed [20,21,22,23,24,25,26,27,28].
The cost of computations can be linearized using fast multipole method (FMM) and
sparse matrix techniques for certain kinds of calculations [29,30,31,32,33,34].
of the atomic orbital (AO) integrals to the molecular orbital basis by "in-core"
means (storing the AO integrals in memory), "direct" means (no integral storage
required), "semi-direct" means (using some disk storage of integrals), or "conventional"
means (with all AO integrals on disk).
Use of density fitting to
speed up the Coulomb part of pure DFT calculations [35,36].
quadrature to compute DFT XC energies and their derivatives.
Molecular mechanics calculations using the AMBER ,
DREIDING  and UFF [39,40]
Semi-empirical calculations using the CNDO ,
INDO , MINDO/3 [43,44],
and PM3 [55,56] model
Self-consistent field calculations using closed-shell
(RHF) , unrestricted open-shell (UHF) ,
and restricted open-shell (ROHF)  Hartree-Fock
Correlation energy calculations using Møller-Plesset
perturbation theory  carried to second, third
, fourth [62,63],
or fifth order. MP2 calculations use direct
[21,65] and semi-direct
methods  to use efficiently however much (or
little) memory and disk are available.
Correlation energy calculations
using configuration interaction (CI), using either all double excitations (CID)
or all single and double excitations (CISD) .
cluster theory with double substitutions (CCD),
coupled cluster theory with both single and double substitutions (CCSD) [68,69,70,71],
Quadratic Configuration Interaction using single and double substitutions (QCISD)
, and Brueckner Doubles Theory (BD) [73,74].
A non-iterative triples contribution may also be computed (as well as quadruples
for QCISD and BD).
Density functional theory [75,76,77,78,79],
including general, user-configurable hybrid methods of Hartree-Fock and DFT. See
this page for a complete list of available functionals.
high accuracy energy methods: G1 theory [80,81],
G2 theory , G2(MP2) 
theory, G3 theory , G3(MP2) ,
and other variants ; Complete Basis Set (CBS)
methods: CBS-4 [91,92],
CBS-q , CBS-Q ,
and CBS-QCI/APNO , as well as general CBS extrapolation;
the W1 method of Martin (with slight modifications) [94,95,96].
MCSCF, including complete active space SCF (CASSCF) [97,98,99,100],
and allowing for the optional inclusion of MP2 correlation .
Algorithmic improvements  allow up to 14 active
orbitals in Gaussian 03. The RASSCF variation is also supported
Generalized Valence Bond-Perfect Pairing (GVB-PP) SCF method .
the SCF wavefunctions for stability under release of constraints, for both Hartree-Fock
and DFT methods [106,107].
state energies using the single-excitation Configuration Interaction (CI-Singles)
method , the time-dependent method for HF
and DFT [109,110,111],
the ZINDO semi-empirical method [112,113,114,115,116,117,118,119,120],
and the Symmetry Adapted Cluster/Configuration Interaction (SAC-CI) method of
Nakatsuji and coworkers [121,122,123,124,125,126,127,128,129,130,131,132,133,134,135].
and Geometry Optimizations
Analytic computation of the nuclear
coordinate gradient of the RHF , UHF, ROHF,
GVB-PP, CASSCF [137,138],
MP3, MP4(SDQ) [141,142],
CID , CISD, CCD, CCSD, QCISD, Density Functional,
and excited state CIS energies . All of the
post-SCF methods can take advantage of the frozen-core approximation.
geometry optimization to either minima or saddle points [136,144,145,146,147,148],
using internal or cartesian coordinates or a mixture of coordinates. Optimizations
are performed by default using redundant internal coordinates ,
regardless of the input coordinate system used.
state searching using synchronous transit-guided quasi-Newton methods .
path following using the intrinsic reaction coordinate (IRC) [151,152].
or three-layer ONIOM [153,154,155,156,157,158,159,160,161,162,163]
calculations for energies and geometry optimizations.
optimization of a transition state and a reaction path .
intersection optimization using state-averaged CASSCF [165,166,167].
calculation which locates the point of maximum energy for a transition structure
along a specified reaction path [168,169,170,171,172,173,174,175,176].
Classical trajectory calculation in which the classical equations
of motion are integrated using analytical second derivatives [177,178,179,180]
Born Oppenheimer molecular dynamics (BOMD) [177,178,179,180,181,182]
(see  for a review) [184,185,186,187,188].
This can be done using any method for which analytic gradients are available,
and can optionally make use of Hessian information.
of the electronic degrees of freedom via the Atom Centered Density Matrix Propagation
molecular dynamics model [188,189,190].
This method has similarity and differences to the related Car-Parrinello approach
. See the discussion of the ADMP
keyword for details. This can be done using the AM1, HF, and DFT methods.
and Second Derivatives
Analytic computation of force constants
(nuclear coordinate second derivatives), polarizabilities, hyperpolarizabilities,
and dipole derivatives analytically for the RHF, UHF, DFT, RMP2, UMP2, and CASSCF
and for excited states using CIS.
Numerical differentiation of
energies or gradients to produce force constants, polarizabilities, and dipole
derivatives for the MP3, MP4(SDQ), CID, CISD, CCD, and QCISD methods [143,200,201,202].
vibrational analysis and thermochemistry analysis using arbitrary isotopes, temperature,
Analysis of normal modes in internal coordinates.
of IR and Raman intensities for vibrational transitions [193,194,196,200,203].
Pre-resonance Raman intensities are also available.
vibration and vibration-rotation coupling [204,206,207,208,209,210,211,212,213,214].
Anharmonic vibrations are available for the methods for which analytic second
derivatives are available.
of various one-electron properties using the SCF, DFT, MP2, CI, CCD and QCISD
methods, including Mulliken population analysis ,
multipole moments, natural population analysis, electrostatic potentials, and
electrostatic potential-derived charges using the Merz-Kollman-Singh [216,217],
CHelp , or CHelpG 
Static and frequency-dependent polarizabilities and hyperpolarizabilities
for Hartree-Fock and DFT methods [220,221,222,223,224,225].
shielding tensors and molecular susceptibilities using the SCF, DFT and MP2 methods
Susceptibilities can now be computed using GIAOs [236,237].
Spin-spin coupling constants can also be computed [238,239,240,241]
at the Hartree-Fock and DFT levels.
Vibrational circular dichroism
(VCD) intensities , and Raman optical activity [579, 583, 584].
methods for electron affinities and ionization potentials [243,244,245,246,247,248,249].
spin orbit coupling between two spin states can be computed during CASSCF calculations
Electronic circular dichroism [255,256,257,258,259]
(see  for a review).
rotations and optical rotary dispersion via GIAOs [261,262,263,264,265,266,267,268,269,270,271].
spectra: g tensors, nuclear electric quadrupole constants, rotational constants,
quartic centrifugal distortion terms, electronic spin rotation terms, nuclear
spin rotation terms, dipolar hyperfine terms, and Fermi contact terms [272,273,274,275,276,277,278,279].
Input can be prepared for the widely used program of H. M. Pickett .
All of these models employ a self-consistent reaction field (SCRF)
methodology for modeling systems in solution.
Onsager model (dipole
and sphere) [281,282,283,284],
including analytic first and second derivatives at the HF and DFT levels, and
single-point energies at the MP2, MP3, MP4(SDQ), CI, CCD, and QCISD levels.
Continuum (overlapping spheres) model (PCM) of Tomasi and coworkers [285,286,287,288,289,290,291,292,293,294,295,296,297,298,299,300,301,302,303]
for analytic HF, DFT, MP2, MP3, MP4(SDQ), QCISD, CCD, CCSD, CID, and CISD energies
and HF and DFT gradients and frequencies.
can be computed for excited states [298,299,300].
properties can be computed in the presence of a solvent [304,305,306].
IPCM (static isodensity surface) model 
for energies at the HF and DFT levels.
isodensity surface) model  for analytic energies
and gradients and numerical frequencies at the HF and DFT levels.