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Development of New Code Introduction
All the
simulation codes used in the group were written or adapted by J.L.Martins or his
students and close collaborators. Many groups throughout the world also use
them. Writing new codes, improving or maintaining the old codes is an ongoing
effort. There are four major codes:
1)
Electronic structure of atoms with pseudopotential generation;
2)
Electronic structure of molecules with a gaussian basis set;
3)
Electronic structure of crystals with a plane-wave basis set;
4)
Micromagnetic simulation with finite differences.
The
pseudopotential generation code is very stable, and in recent years it has only
been improved in the interface with the user, plotting, analysis and test of the
results, new options. The program is available on the web
http://bohr.inesc.pt/~jlm/pseudo.html. It generates the
"Troullier-Martins" pseudopotential (Phys. Rev. B 43, 1993, 1991).
From the citations to that article it can be seen that it is widely used and
therefore it is important to maintain a bug-free easy to use code. A graphical
interface to the program will be available soon.
The molecular
program is relatively old (J.Chem.Phys. 78, 5646, 1983), in fact it was one of
the first density functional programs for molecules. Today there are commercial
and academic packages using the same methods which were developed by large
groups, so it is only modified when a new problem demands it. That occurred in
1999, when we used that old code as a basis of a first principles Monte Carlo
code. Statistical mechanical simulations are traditionally done with molecular
dynamics, requiring the calculation of forces on ionic cores, or Monte-Carlo,
requiring only the calculation of the total energy. The trade-off is that a
Monte-Carlo simulation requires the calculation of a much larger number atomic
configurations. For empirical methods both methods were competitive, however in
first principles methods the calculations of forces is a minor effort compared
with the calculation of the total energy, and therefore we are only aware of
first principles molecular-dynamics simulations (pioneered by Car and
Parrinello). In a molecular code with a gaussian basis set, most of the
computing time is spent calculating the so-called molecular integrals. In a
typical Monte-Carlo simulation only one atom is moved from one configuration to
the next. In that case most integrals remain the same, more precisely for an
N-atom molecule only 1/N of the integrals change value. We used that property,
plus an efficient paralelization of the calculation of the molecular integrals
to develop a first-principles density-functional Monte-Carlo simulation code
that can be used for large molecules. As far as we know, it is the first code of
that kind.
The
plane-wave code for crystals (Phys.Rev. B37, 6134 (1988)) has been the most
important in the past research. We were the first to present principles
molecular-dynamics simulations without using the Car-Parrinello algorithm (Solid
State Commun. 78, 831, 1991), and were the first to do a first principles
simulation with an optimization of the shape of the crystal cell (Phys. Rev.
Lett. 70, 3947, 1993). Recent developments include a new algorithm to optimize
the cell shape. In 1999 the code suffered a major upgrade with the addition of
new options (for example the GGA density functional), consolidation of
non-compatible versions developed by close collaborators, and improvements in
computational speed. The new version is now being used for research, and once we
document the code, we plan to make it available on the web (the old version was
distributed on a informal basis only).
In 1998 a
micromagnetics code was written from scratch. It uses a finite differences
method to solve the magnetic equations and a molecular dynamics simulated
annealing to minimize the magnetic energy. It is quite fast and we are testing
it on the "standard problems". We missed by a week being the first to
publish a solution to the "third problem". A new version of the code
has been completed recently. Paralelization is under way. For more information on this research topic, contact Prof. José Luís Martins. Main Results
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