Wednesday, July 8, 2015

Gamess (US) frequently asked questions. Part 7: How to distinguish alpha from beta orbitals in the $VEC deck

Each line in a $VEC group contains the coefficients of five basis functions for a given orbital. These are formatted in a special way, with seven numbers in each line. These numbers are:

1st) the number of the orbital to which the coefficients belong (written with at most two characters, so that 1 means orbital 1, .. , 99 means orbital 99, 00 means orbital 100) . This number is repeated in the beginning of every line, until all coefficients for that orbital have been written

2nd) this number tells the program how to assign the coefficients to the basis functions. "1" means that the coefficients are for basis functions 1-5, "2" means that the coefficients are for basis functions 5-10, etc. In general , that number "n" directs the program to assign the five coefficients present in the line to basis functions 5*(n-1)+1 to 5*n.

3rd to 7th) coefficients of five basis functions

BETA orbitals are punched as a group immediately after all ALPHA orbitals.

This format entails that in molecules with more than 100 orbitals the $VEC group contains several blocks with the same 1st number. For example, in a molecule with 200 orbitals, alpha orbital 27 is described by the first block of lines beginning with "27", and alpha orbital 127 is described by the SECOND block of lines beginning with "27".

I usually find the beginning of the BETA orbitals by repeating a search for the string " 1 1" : if that string is preceded by a block beginning with "00 1", it usually refers to orbitals 101, or 201, etc. (the exception being those systems with exactly 100, 200, etc. orbitals). If string " 1 1" is NOT preceded by a block beginning with "00 1", you are sure to have found the beginnning of the BETA orbitals

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