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The Molecular
ADF Program
Geometry
optimizations, transition states, and
reaction paths
- ADF enables geometry optimizations
in Cartesian and internal coordinates.
- An initial Hessian estimate speeds
up the optimizations.
- Various constraints can be imposed.
- Transition state searches,
intrinsic reaction coordinates, and
linear transit calculations are
available to further analyze the energy
path from reactants via the transition
state to the final products.
- Finite difference and analytic
second derivatives yield IR frequencies
and Hessians. These Hessians are
helpful in finding and characterizing
transition states.
Molecular
properties with ADF
- The time-dependent DFT
implementation yields UV/Vis spectra
(singlet and triplet excitation
energies, as well as oscillator
strengths), frequency-dependent
(hyper)polarizabilities (nonlinear
optics), Raman intensities, and van der
Waals dispersion coefficients.
- Rotatory strengths and optical
rotatory dispersion (optical properties
of chiral molecules) are being
implemented (contact us for the current
status).
- Frequency-dependent dielectric
functions for periodic structures will
soon become available (contact us for
the current status).
- NMR chemical shifts and spin-spin
couplings are available.
- ESR (EPR) g-tensors, magnetic and
electric hyperfine tensors, and nuclear
quadrupole coupling constants can be
treated.
- Standard properties like IR
frequencies and intensities, and
multipole moments are of course
accessible.
- Relativistic effects (ZORA and
spin-orbit coupling) can be included
for most properties.
Analysis
ADF contains several unique analysis
options to help a gain detailed
understanding of the chemical problem at
hand. These methods highlight the
underlying philosophy that the Kohn-Sham
orbitals in DFT can be used for a
"quantitative MO theory".
Molecule built from fragments
ADF and BAND analyze their results in
terms of user-specified subsystems from
which the total system is built. The
program offers the "fragment orbitals"
(FOs) of the chemically meaningful
sub-units mix with FOs on other fragments
to combine to the final molecular
orbitals.
Bond energy analysis
ADF calculates various chemically
meaningful terms that add up to the bond
energy, with an adaptation of Morokuma's
bond energy decomposition to the Kohn-Sham
MO method. The individual terms are
chemically intuitive quantities such as
electrostatic energy, steric repulsion,
Pauli repulsion, and orbital interactions.
The latter are symmetry decomposed
according to the Ziegler transition state
method.
Advanced charge density analysis
In addition to Mulliken charge
analysis, ADF calculates several atomic
charges that do not share the flaws of
Mulliken (strong basis set dependence).
These charge analysis methods ("Voronoy
deformation density" and "Hirshfeld")
provide atomic charges that agree well
with chemical intuition.
Molecular symmetry
ADF uses the full molecular symmetry
including non-Abelian groups. The proper
symmetry labels to orbitals, excitations,
vibrational modes are provided on output.
Accuracy
ADF embodies a set of unique technical
features that ensure reliable and accurate
calculations.
Slater type basis sets
ADF uses Slater Type Orbitals (STOs) as
basis functions. These resemble the true
atomic orbitals more closely than the more
common Gaussian Type Orbitals (GTOs).
Therefore, fewer STOs than GTOs are needed
for a certain level of accuracy. ADF has a
database with thoroughly tested basis set
files, ranging in quality from single-zeta
to quadruple zeta basis sets with various
diffuse and polarization functions. They
are available for all elements, including
lanthanides and actinides. In the BAND
program, numerical atomic orbitals are
used in addition to Slater type orbitals.
Integration scheme
ADF and BAND use the unique Te
Velde-Baerends [6] numerical integration
scheme, in which the grid is automatically
adapted to the available basis functions
and to the number of significant digits
demanded by the user through a single
input parameter. It is straightforward to
do very accurate integrations with much
fewer points than in less highly developed
schemes.
Transition metal compounds and heavy
elements
Users recommend ADF for its ability to
provide the same stability for complex
transition metal compounds as for simpler
systems containing only light atoms. The
relativistic methods and basis sets in ADF
enable treatment of molecules with very
heavy elements.
Modern xc energy functionals and
potentials
A variety of the most accurate modern
(meta-)GGA exchange-correlation (xc)
energy functionals are all evaluated
simultaneously in ADF. For reliable
property calculations, improved xc
potentials with correct asymptotic
behavior, such as SAOP and GRAC, are
available in ADF.
Efficiency,
treatment of large molecules
One of the main complications that can
arise in chemically relevant applications
of DFT software is the treatment of large
molecules. ADF has several qualities to
enable treatment of such systems.
QM/MM
For truly large system sizes (more than
a few hundred atoms), a mix of quantum
mechanics and molecular mechanics (QM/MM)
is often suitable if the major quantum
effects are restricted to a certain part
of the molecule ("active site"). QM/MM
calculations can be performed on much
larger systems than pure QM calculations,
because the approximate MM calculations
are very fast. Various standard force
fields (Sybyl, Amber, UFF) are available.
Parallelization
Most parts of ADF have been efficiently
parallelized for both shared-memory and
distributed memory systems, such as simple
Linux clusters. For most standard types of
calculation, ADF approaches perfect linear
scaling fairly well, even for a
significant amount of CPUs.
Linear scaling / distance cut-offs
Because of the exponential spatial
decay of the STO basis functions, ADF can
easily exploit the fact that atoms that
are far apart do not interact. This
reduces the computational complexity from
O(Natom 3) to O(Nat) for the most
time-consuming parts of the calculation,
leading to dramatic savings.
Density fit and frozen core
approximation
A density fit procedure reduces the
cost of the Coulomb potential evaluation.
A frozen core approximation can be used to
considerably reduce the computation time
for systems with heavy nuclei, in a
controlled manner.
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