This "basis set completeness" problem is the basis set superposition error (BSSE), which has been thoroughly studied in molecular calculations. All methods based on atom centred basis sets suffer from it. It basically leads to an artificial attractive force between atoms, because in a more compact system, each atom is described with a more complete basis. In the context of periodic calculations, I would particularly expect it to overestimate surface adsorption energies and underestimate bond lengths.



?????? ????? wrote:
Dear Siesta users,

recently Andrei Postnikov wrote a small tutorial on Siesta

He wrote:
"one cannot a priori expect equally good performance of basis over
a large range of bond lengths and, say, pressures: as the pressure is varying, the “completeness” or “variational freedom” of basis in the region of chemically relevant, under a given pressure, bond lengths do vary; consequently the calculated pressure/energy curve may slightly differ from that
obtained in a “benchmark” FLAPW calculation."

A good test would be to compare bulk VASP and Siesta calculations and deside if calculation in Siesta is accurate enough at some particular pressure. Unfortunately we don't have VASP here.

My question is (+/- 10%) of the equlibrium lattice constants of Si and Ge are Ok for Siesta? Will be Siesta accurate enough to calculate surface energy with an error of about 1 meV/A2 ?

Kind regards
Ruslan Zhachuk

Herbert Fruchtl
Senior Scientific Computing Officer
School of Chemistry, School of Mathematics and Statistics
University of St Andrews
The University of St Andrews is a charity registered in Scotland:
No SC013532

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