Ángel Martín Pendás
Quantum Chemistry Group

This page gathers a certain amount of information concerning my professional and not so professional interests. I'm a 31 Professor of Chemical Physics at the University of Oviedo, a ~200000 people city in the north of Spain. Unfortunately, most internet resources about this evergreen rainy region are written in spanish . Our University was founded in 1579 and holds a rich and tortuous history that you may be interested in.
My research activities are centered around the theoretical simulation of the electronic structure of crystalline materials using the methods provided by Quantum Mechanics. Along the years, our group has developed powerful computational methods based on the Theory of Electronic Separability that solve the HartreeFock (HF) equations of a solid in a localized Fock space. The resulting scheme, named the ab initio Perturbed Ion method (Comp. Phys. Comm. 77 107134 (1993) ), is able to compete advantageously with standard techniques. It is currently several orders of magnitude faster than other HF programs, and may be obtained at no cost for nonprofit purposes. Contact me if you are interested in it, or if you simply want some type of technical information. As a byside effect of my everyday work, I'm also really interested in computational problems. I usually develop my software applications on a i486100 PC running Linux. If you haven't heard (really?) about the niceties of this transformyourpcintoapowerfulworkstation operating system, now it is a good moment to get more information in this same server.
The rest of the page is a collection of brief summaries of some of my works, together with some pointers to interesting web places. If you are looking for really technical information, you can pick up some of our papers
The main difficulty that Quantum Chemistry, as a discipline, has found in the main stream chemical community is rooted on the definition domain of the molecular wave function: the whole 3D space. This is the famous Hilbert space problem, and prevents that properties depending on a localized region of space be rigourously quantum mechanically defined. On the other hand, Chemistry is inseparable from local concepts, and much work has been done along the years to overcome this difficulty. None of the large number of approaches that have been proposed, except that of Prof. Bader (R. F. W. Bader, Atoms in Molecules: A Quantum Theory,(Oxford U. P., 1990)), is fully coherent from the theoretical point of view.
Bader's main concept is a theorem proving that non relativistic quantum mechanics is applicable to open regions of the 3D space if these regions are bounded by surfaces whose flux of the gradient of the electronic density vanishes. In this way, topological atoms may be defined. Any quantummechanical observable is then found to be the addition of the local expected values over the basins of the topological atoms.
The periodicity of a crystalline material changes a little bit the usual panorama of the theory. We have found new topological objects not previously defined and obtained very interesting connections between the topology of the electron density of a crystalline material and its energetic stability. More information can be found from Bader's own web Homepage, one of my colleagues page, one of our web papers, or a recent communication to the watoc conference
As an example of the kind
of topological atoms that
are obtained in a perovskite crystal, KCaF_{3}, the figure on the left
shows them superimposed on the crystal lattice. The gold objects are the calcium
atoms. Notice how their wings envelope almost completeley the fluorines, in
green. The potasium atoms are shown in red. The
electronic densities were
obtained with the aiPI
program. The topological basins were then
assembled with our AIM package (the CRITIC
program). Finally,
the surfaces were obtained with our TESSEL
code, and rendered
with POVRAY
. Contact me about this codes for any question.
The simulation of the thermodynamical properties of solid bodies by means of both Molecular Dynamics or MonteCarlo techniques has always rested on the availability of suitable pair potentials feeding the calculations. It has only been very recently when Carr & Parrinello schemes have allowed true quantum mechanical simulations of materials, always from the density functional theory perspective. It is therefore very important that good, tested potentials be available to the scientific community.
We have devised a collection of methods to obtain reliable pair potentials
from our quantum mechanical procedures. A general purpose code, named
PAIRPOT
, has been constructed to study the energetic and thermodynamic
features of isolated clusters and pure or impurified crystals. The code
accepts both numerical and/or analytic potentials, and has a builtin
calculator that allows the user to define the geometry of the system examined
in terms of freely configurable variables. Space group symmetry is explicitly
taken into account, in such a way that crystal structures may be introduced
directly from databases. The program optimizes or differentiates several
aim functions, like the total, Helmholtz or Gibbs energies, simplifying
considerably the analysis of results. It has also several builtin graphical
capabilities. The code,
freely available for nonprofit purposes, is still in a beta stage.
Here there are some optimized geometries of small clusters of NaCl molecules,
Na_{n}Cl_{n}, where n ranges from 3 to 9. The calculations
were made in my desktop PC with aiPI derived pair potentials. The pictures
were rendered directly from PAIRPOT
with the aid of a modified
version of Roger Sayle's RASMOL
Below this lines you will find the simple input file that
optimizes and generates the picture corresponding to the
Na_{3}Cl_{3} cluster

It is equally easy to obtain simple pictures of crystalline structures,
like this one of the rocksalt phase of NaCl, that may be manipulated
interactively, and transformed automatically to high quality
raytracing images.
The PAIRPOT
code
is also able to compute thermodynamic properties
at non zero temperature at both the rigid and quasi harmonic approximations.
The vibrational Helmoltz energy is obtained from the phonon frecuencies
calculated automatically for any kind of pair potentials, analytic or numeric in
nature. The dynamical matrix is not added up to first, second, or nth
neighbours. On the contrary, every atom in the lattice makes its contribution
until complete convergence is achieved. In this way, every module in the
program is consistent with the general aim of the code. The following figure
shows the total and projected density of vibrational states obtained for the
NaCl crystal at the theoretical equilibrium distance using the same potentials
as those of the molecular clusters examined above.
The code that generates this figure is also shown.

As a final example of the actual capabilities of our code, you can enjoy the following display of the isofrequency surfaces of each phonon branch in the above NaCl crystal. Frequencies are given in cm^{1}. Branches are labelled in rows. Aren't they worth the work?
Frequency  1  2  3  4  5  6 
60  
80  
100  
120  
140  
160  
180  
200  
220  
240  
260  
280 
APS Editorial
Offices. The American Physical Society Editorial Offices.
OUP Oxford University
Press
Springer
El PAIS. Perhaps the
best Spanish daily newspaper.
El MUNDO. Another
spanish paper. Good web resources.
PIC. Spanish Ministery
of Culture information service. Good Gastronomy!
BOE. Spanish daily Official
Bulletin.
Several
Spanish Government services
National Spanish
Library
AstroWeb
Astronomy/Astrophysics on the Internet.
Planet Finder
Use it.
NASA HomePage. Don't
forget it. It's a good starting point.