accurate forces for nonperiodic calculation

[Matt W]

Hi Noam,

I'd start with using the PBE functional, as it is numerically a lot more 
stable than BLYP. I don't have much experience with BLYP/smoothing ...

Taking a water molecule in vacuum with PBE, forces converge smoothly with 
cutoff  - results below are for the input attached and cutoffs of 
400,600,800 and 1000 Ry. I've not explicitly checked the 
translational/rotational invariance, but the sum of forces is getting very 
reasonable, which probably is a good hint of things being OK.

HTH,

Matt

mattw at mattw:~$ grep -A 6 ' ATOMIC FORCES in ' H2O-geoopt_???.out 
H2O-geoopt_????.out
H2O-geoopt_400.out: ATOMIC FORCES in [a.u.]
H2O-geoopt_400.out-
H2O-geoopt_400.out- # Atom   Kind   Element          X              Y       
       Z
H2O-geoopt_400.out-      1      1      O           0.00000009     
0.00000003    -0.01711982
H2O-geoopt_400.out-      2      2      H          -0.00000004   
 -0.01005315     0.00720393
H2O-geoopt_400.out-      3      2      H          -0.00000004     
0.01005312     0.00720394
H2O-geoopt_400.out- SUM OF ATOMIC FORCES           0.00000001   
 -0.00000000    -0.00271194     0.00271194
--
H2O-geoopt_600.out: ATOMIC FORCES in [a.u.]
H2O-geoopt_600.out-
H2O-geoopt_600.out- # Atom   Kind   Element          X              Y       
       Z
H2O-geoopt_600.out-      1      1      O           0.00000008     
0.00000002    -0.01357588
H2O-geoopt_600.out-      2      2      H          -0.00000004   
 -0.01004780     0.00719010
H2O-geoopt_600.out-      3      2      H          -0.00000004     
0.01004777     0.00719010
H2O-geoopt_600.out- SUM OF ATOMIC FORCES           0.00000001   
 -0.00000000     0.00080432     0.00080432
--
H2O-geoopt_800.out: ATOMIC FORCES in [a.u.]
H2O-geoopt_800.out-
H2O-geoopt_800.out- # Atom   Kind   Element          X              Y       
       Z
H2O-geoopt_800.out-      1      1      O           0.00000006     
0.00000002    -0.01405749
H2O-geoopt_800.out-      2      2      H          -0.00000003   
 -0.01004908     0.00718143
H2O-geoopt_800.out-      3      2      H          -0.00000003     
0.01004906     0.00718144
H2O-geoopt_800.out- SUM OF ATOMIC FORCES           0.00000000   
 -0.00000000     0.00030538     0.00030538
--
H2O-geoopt_1000.out: ATOMIC FORCES in [a.u.]
H2O-geoopt_1000.out-
H2O-geoopt_1000.out- # Atom   Kind   Element          X              Y     
         Z
H2O-geoopt_1000.out-      1      1      O           0.00000006     
0.00000002    -0.01445572
H2O-geoopt_1000.out-      2      2      H          -0.00000003   
 -0.01004982     0.00718060
H2O-geoopt_1000.out-      3      2      H          -0.00000003     
0.01004980     0.00718060
H2O-geoopt_1000.out- SUM OF ATOMIC FORCES           0.00000000   
 -0.00000000    -0.00009452     0.00009452



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