Wednesday, June 19, 2013

Gamess (US) frequently asked questions Part 1: SCF convergence

In spite of the very high quality of the Gamess(US) documentation, the Gamess(US) list is very often flooded with requests from new users regarding the lack of convergence of the SCF procedure. A few words of advice:

When your SCF does not converge,  you should re-run the job including a $guess guess=moread $end line, as well as the complete $VEC group present in the output PUNCH file (usually called <jobname>.dat, and present in you scratch directory).


    Whenever you read a $VEC group from a UHF run you must assign NORB in the $GUESS group. An additional problem is that by default the $VEC group only includes the occupied orbitals, and this means that in UHF runs the $VEC group does not include equal numbers of alpha and beta orbitals (e.g., a run with 41 electrons and MULT=2) will have 21 alpha orbitals and 20 beta orbitals. Therefore, if you include

    $guess guess=moread NORB=21 $end

    Gamess will crash because there are not 21 beta orbitals, and if you input

    $guess guess=moread NORB=20 $end

    there will be another error, since there are more than 20 alpha orbitals. In these cases, you should check the number of alpha and beta orbitals. Then , copy the coefficients of the extra alpha orbitals to the end of the beta orbitals. In my example above

    $guess guess=moread NORB=21 $end

    will yield no problems, since the modification of the VEC group yields equal numbers of alpha and beta orbitals. There is also an option to PUNCH every orbital (occupied+virtuals) at every step. In this case, Gamess always punches a full $VEC group, making it very easy to assign NORB as one can simply inspect the output file to learn the number of orbitals. However, this yields gigantic PUNCH files, and may therefore not be feasible.

You should also experiment with changing convergers, damping, etc. Some systems are notoriously hard to converge, and may require several re-iterations of the whole process. 


  1. Thank you so much for sharing the tip! I am having the exact problem of importing $VEC and run with UHF. More specifically, I have 55 ALPHA and 54 BETA orbitals. The thing is, I can find ALPHA and BETA orbitals in the output log, but cannot really distinguish them in the .dat file $VEC section. Could you give me some hints on how to find the extra alpha orbitals, maybe some examples? Thank you!

  2. 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

    1. Thank you so much for the very clear and detailed explanation! It really helps! Now I am able to run with MOREAD for UHF jobs. :)