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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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