unusual slow calculation with larger systems

[frontleg]

Hi, All,

We tried to compile the vasp 5.2 and 5.3 on new linux cluster (http://www.rcac.purdue.edu/userinfo/resources/carter/). It worked quite well at the beginning. But after we increase the system size (e.g. from non-spin polirized MgO to spin polirized and +U corrected FeO; from small vacuum to large vacuum). The calculation became extremely slow in some cases. In such slow cases, OUTCAR file showed that the "EDDIAG:" and "ORTHCH:" time is much longer (hundreds seconds and even > 1000s) than it in the normal case (< 10s). The "RMM-DIIS:" time do not have a big different in all the cases.

We checked the memory usage and intra-nodes communications and it turned out no limitations from the hardware.

Could anyone give us some hint on how to solve this problem?

Thank you in advance!

zhao

[admin]

Expanding the vacuum is not "free of charge." The number of plane waves scales with the cell volume. With increasing volume the PW basis set increases considerably and this leads to the increase of the computer time.

[frontleg]

Thank you very much for the reply. I am sorry that I did not explain my problem very clear.

My problem is the computational time is randomly slow when we have a larger system. For example, in one of my calculation, in the first SCF　LOOP, the speed is normal, e.g. the time consuming for one electronic step is as following:

POTLOK: cpu time 0.85: real time 0.91
SETDIJ: cpu time 1.73: real time 1.73
EDDIAG: cpu time 9.22: real time 9.27
RMM-DIIS: cpu time 13.58: real time 13.61
ORTHCH: cpu time 4.12: real time 4.13
DOS: cpu time 0.00: real time 0.00
CHARGE: cpu time 0.69: real time 0.69
MIXING: cpu time 0.01: real time 0.01
--------------------------------------------
LOOP: cpu time 30.19: real time 30.38

However, from the second SCF LOOP, the calculation became very slow, here is the time for one electronic step:

POTLOK: cpu time 1.47: real time 1.47
SETDIJ: cpu time 1.72: real time 1.72
EDDIAG: cpu time 380.17: real time 381.03
RMM-DIIS: cpu time 12.35: real time 12.38
ORTHCH: cpu time 46.10: real time 46.21
DOS: cpu time 0.00: real time 0.00
CHARGE: cpu time 40.64: real time 40.73
MIXING: cpu time 0.01: real time 0.01
--------------------------------------------
LOOP: cpu time 482.47: real time 483.63

Note that the RMM-DIIS time is nearly the same in these two cases, but others like EDDIAG, ORTHCH, CHARGE became very slow.

We think it might not be the problem of the hardware. So we are wondering whether it is the problem with the compiling of vasp or other related programs.

Could anyone give us some hint on it?

Thank you in advance!

zhao