Introduction

VAMP/VASP is a complex package for performing ab-initio quantum-mechanical molecular dynamics (MD) using pseudopotentials and a plane wave basis set. The approach implemented in VAMP/VASP is based on a finite-temperature local-density approximation (with the free energy as variational quantity) and an exact evaluation of the instantaneous electronic ground state at each MD-step using efficient matrix diagonalization schemes and an efficient Pulay/Broyden mixing. These techniques avoid all problems occurring in the original Car-Parrinello method which is based on the simultaneous integration of electronic and ionic equations of motion. The interaction between ions and electrons is described using ultrasoft Vanderbilt pseudopotentials (US-PP). These pseudopotentials allow a considerable reduction of the necessary number of plane-waves per atom for transition metals and first row elements. Forces and the full stress tensor can be easily calculated with VAMP/VASP and used to relax atoms into their instantaneous groundstate.Here is a short summary of the highlights of VAMP/VASP :

• As already mentioned VAMP/VASP uses US-PP. Therefore the size of the basis-set can be kept very small even for transition metals and first raw elements like C and O. Generally not more than 100 plane waves (PW) per atom are necessary to describe a system, in most cases even 50 PW per atom will be sufficient.
• There are always some parts in a plane wave code, which scale as , N being the number of the atoms in the system. One of the highlights of VAMP/VASP is that these parts are almost negligible leading to an efficient scaling in terms of computer time. This is possible by evaluating the non local contributions of the pseudopotential in real space and by keeping the number of orthogonalizations small.
• VAMP/VASP uses a rather traditional self-consistency scheme to calculate the electronic groundstate. But in our case traditional does not mean slow. Actually, we have taken the best of the old ideas and combined them with efficient new numerical methods leading to an extremely efficient and fast scheme for evaluating the selfconsistent solution of the Kohn-Sham functional. The used iterative matrix diagonalization schemes must be considered the best currently around. For complex surface calculations the Pulay/ Broyden-mixing scheme speeds up self-consistent calculations by almost an order of magnitude.
• VAMP/VASP runs equally well on super scalar processors and vector computers. For parallel computers a version will be release soon. Ports for the following machines exist:
  • IBM RS6000
  • HP 700 Series (PA-RISC), and other models
  • SGI Power Challenge
  • SUN
  • Fujitsu vector machines
  • Cray vector machines
  • Cray T3D (vasp.4.2 only)
  • IBM-SB2 (vasp.4.2 only)
  (for a performance profile of these machines look at Section 3.9).
• VAMP/VASP includes a full featured symmetry code which determines the symmetry of arbitrary configurations automatically.
• The symmetry code is also used to set up the Monkhorst pack special points allowing an efficient calculation of bulk materials, symmetric clusters etc. The integration of the bandstructure energy over the Brillouin zone is performed using these special points and tetrahedron methods. The Bloechl corrections which remove the quadratic error of the linear tetrahedron method result in a fast convergence speed with respect to the number of special points.

  As an alternative to the tetrahedron method, it is also possible to use finite temperature LDA-theory, which is almost as efficient as the tetrahedron method. The finite temperature LDA-theory is especially interesting if the size of the system is so large, that only one or two special k-points can be used.