CamCASP/Bugs/7

Most of the polarizabilities generated by CamCASP show the expected monotonic decrease with increasing imaginary frequency. A few, however, depart wildly from this behaviour. In the example case of formamide, we get, for C,

20 20
            3.74300000      3.98200000      5.26700000      8.33300000 +++
           11.43900000     11.41700000      7.41600000      2.64700000 +++
            0.42400000      0.01400000

with similar behaviour for C 11c 20, C 22c 22s and O 11c 20, but none of the other cases. Removing the "options tests" line from the data to get the standard grid makes things even worse:

20 20
           -9.88200000     -9.52700000     -7.58300000     -2.61900000 +++
            3.70900000      7.16600000      5.79900000      2.32900000 +++
            0.39500000      0.01400000

This is a problem with pfit; the anchor values are

           33.27220340     33.16682559     32.53705396     30.41159247 +++
           25.60290131     18.22199084     10.00593354      3.58000511 +++
            0.63051681      0.02341723

Weight 5 improves matters, but weight 6 is needed to get sensible results: 20 20

           20.80100000     20.83700000     20.99700000     21.01100000 +++
           19.14900000     14.68600000      8.43300000      2.77400000 +++
            0.39500000      0.01400000

The fit is still pretty good: r.m.s. error 0.073% for weight 6 compared with 0.042% for weight 4. Even with weight 6 there are some anomalous cases, especially O 20 20, but the absolute values are small.

Ideally we should force the monotonic decrease in magnitude, but I don't see how to do it, even if we fitted all frequencies together.