In all the O() methods supported by OpenMX Ver. 3.6, neighboring atoms in each truncated cluster are classified into two categories: first and second neighboring atoms. If the sum, , of a cutoff radius, , of basis functions allocated to the central atom and that, , of a neighboring atom is smaller than the distance between the two atoms, then the neighboring atom is regarded as a first neighboring atom, and the other atoms, which does not satisfy the criterion, in the truncated cluster are called the second neighboring atom. The second neiboring atoms are determined by two keywords, orderN.HoppingRanges and orderN.NumHoppings, while the first neiboring atoms are determined by taking account of only information about cutoff radii of basis functions. The numbers of the first and second neiboring atoms determined by the two keywords are shown in the standard output as FNAN and SNAN, respectively. In addition to the use of two keywords for determing FNAN and SNAN, one can directory controll the number, FNAN+SNAN, by the following keyword:
<orderN.FNAN+SNAN 1 60 2 65 3 60 4 50 .. . orderN.FNAN+SNAN>In this specification, the number of row should be equivalent to that of atoms. The first column is a serial number corresponding to the serial number defined in the keyword, 'Atoms.SpeciesAndCoordinates', and the second column is the number of FNAN+SNAN. Then, the first and second neiboring atoms in each truncated cluster are determined by taking account of the distance between the cental atom and neiboring atoms so that the number of FNAN+SNAN can be equivalent to the value provided by the second column. FNAN+SNAN may largely change when unit vectors are changed, leading to sudden change of the total energy as a function of lattice contant. The user definition of FNAN+SNAN is useful to avoid such a case.