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OpenMX Ver. 3.8 uses the regular real space grid for the evaluation of Hamiltonian matrix elements associated with the Hartree potential by the difference charge density and exchange-correlation potential, and solution of Poisson's equation. Thus, the total energy depends on the relative position between atomic coordinates and the regular grid. When one calculates an interaction energy or energy curve as a function of atomic coordinates, it is quite important to keep the relative position in all the calculations required for the evaluation of the interaction energy. In the calculation for one of the structures, you will find 'Grid_Origin' in the standard output which gives x-, y-, and z-components of the origin of the regular grid as:
Grid_Origin xxx yyy zzz
Then, in order to keep the relative position, you have to include the following keyword
'scf.fixed.grid' in your input file
for all the systems in the calculations
required for the evaluation of the interaction energy:
scf.fixed.grid xxx yyy zzz
where 'xxx yyy zzz' is the coordinate of the origin you got in the calculation
for one of the structures. The procedure largely suppresses the numerical error
involved in the use of the regular grid.
In addition, as discussed in the previous subsection 'A tip for calculating the energy curve for bulks', the number of grids should be fixed by the keyword 'scf.Ngrid' when the lattice parameters are also changed in the evaluation of interaction energy.