convergence & cutoff madness

Toon Verstraelen Toon.Ver... at UGent.be
Sun Sep 6 13:10:48 CEST 2009


Dear CP2K users,

I'm running a few basic tests to get a better understanding of the 
relation between SCF convergence and the plane wave cutoff. The table 
below summarizes a series of trivial energy calculations with different 
cutoffs. The input files are attached.

The system in the input is MIL-53-Al, a well-known metal-organic 
framework. To keep things simple, I've used a very minimal basis 
(SZV-GTH). The largest basis set exponent is 8.3744350009, which 
corresponds to a cutoff of 330 Rydberg. (if not mistaken). The 
functional is blyp.

There are two weird things in this table:

1) The lowest energy is not obtained with the highest cutoff. It gets 
worse above 320.

2) For some high cutoff's there is no SCF convergence. For the low 
range, up to 150, this is normal. I would not expect this for a cutoff 
of 330.

Does somebody know what is going on? The input files are rather simple, 
but maybe I've put in something pernicious? With larger basis sets, 
there are more cases where the SCF convergence fails. It becomes 
practically useless, even when a decent initial guess with a smaller 
basis set is constructed.


Number of failed SCF	 10
cutoff [Ry]   energy [kjmol]   time [s]   #iters   time per iter [s]"
    100.0         370146.36      841.59     250        3.37
    110.0           4309.36     1023.83     250        4.10
    120.0           4311.87     1053.45     250        4.21
    130.0           1159.84     1099.50     250        4.40
    140.0            939.17     1166.17     250        4.66
    150.0           2549.48     1206.51     250        4.83
    160.0            804.26      227.56      42        5.42
    170.0            354.60      159.26      29        5.49
    180.0            412.98      177.18      29        6.11
    190.0            685.96     1455.42     250        5.82
    200.0             79.06      199.92      32        6.25
    210.0             79.17      262.35      40        6.56
    220.0             78.54      201.22      28        7.19
    230.0             86.04      218.52      28        7.80
    240.0             63.43      261.85      35        7.48
    250.0             33.38     2067.27     250        8.27
    260.0             34.90     2237.25     250        8.95
    270.0             22.10      225.49      28        8.05
    280.0             14.48      458.72      52        8.82
    290.0              5.22      322.73      32       10.09
    300.0              4.17      251.49      28        8.98
    310.0              6.07      319.30      32        9.98
    320.0              0.00      296.63      29       10.23
    330.0              2.18     2370.99     250        9.48
    340.0              4.32      351.99      28       12.57
    350.0              4.38      398.81      32       12.46
    360.0              4.37      332.36      25       13.29
    370.0              8.01      496.19      39       12.72
    380.0              8.10      368.99      29       12.72
    390.0             13.16      402.97      29       13.90
    400.0             12.49      410.13      29       14.14


cheers,

Toon

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