events:2018_summer_school:converging_cutoff
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exercises:2018_uol_school:converging_cutoff [2018/05/31 10:02] – [Running the system] mwatkins | exercises:2018_uol_school:converging_cutoff [2018/05/31 14:44] – [Input files] mwatkins | ||
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This exercise is similar to the previous one, but uses a setup and system more typical of CP2K usage. We will use a system of 32 H< | This exercise is similar to the previous one, but uses a setup and system more typical of CP2K usage. We will use a system of 32 H< | ||
- | < | + | < |
&GLOBAL | &GLOBAL | ||
PRINT_LEVEL MEDIUM | PRINT_LEVEL MEDIUM | ||
Line 91: | Line 91: | ||
Compared to the Si example, this is a larger system, we are using the OT optimizer in a good setup for a small to medium insulating system: | Compared to the Si example, this is a larger system, we are using the OT optimizer in a good setup for a small to medium insulating system: | ||
- | < | + | < |
&SCF | &SCF | ||
SCF_GUESS RESTART | SCF_GUESS RESTART | ||
Line 109: | Line 109: | ||
and we are also saving the forces on the atoms | and we are also saving the forces on the atoms | ||
- | < | + | < |
&FORCES | &FORCES | ||
Line 162: | Line 162: | ||
Compare the convergence of forces to the default convergence criteria for geometry optimization. | Compare the convergence of forces to the default convergence criteria for geometry optimization. | ||
+ | |||
+ | What sets the required cutoff? It is the basis set (which is dictated by the pseudopotentials). You will need to be able to represent the Gaussian with largest exponent well on the realspace grids. Oxygen, being very electronegative (on the right of the period table with many protons) has very contracted 2s states. You can see in the output | ||
+ | |||
+ | < | ||
+ | | ||
+ | |||
+ | Set | ||
+ | |||
+ | 1 | ||
+ | | ||
+ | | ||
+ | | ||
+ | | ||
+ | </ | ||
+ | |||
+ | That there is a Gaussian with an exponent of 10.4 Bohr< | ||
+ | |||
+ | < | ||
+ | Si DZVP-MOLOPT-GTH DZVP-MOLOPT-GTH-q4 | ||
+ | 1 | ||
+ | 2 0 2 6 2 2 1 | ||
+ | 2.693604434572 | ||
+ | 1.359613855428 -0.283798205000 | ||
+ | 0.513245176029 -0.228939692700 -0.276015880000 | ||
+ | 0.326563011394 | ||
+ | 0.139986977410 | ||
+ | 0.068212286977 | ||
+ | </ | ||
+ | |||
+ | we see that the largest exponent is only 2.7 Bohr< | ||
+ | |||
+ | < | ||
+ | If you like, have a look at the BASIS_MOLOPT file (in the data directory, or online [[https:// | ||
+ | </ | ||
+ | |||
+ | The convergence is largely dominated by the calculation of the gradient terms in a GGA functional (compare a simulation with LDA to the PBE used here). The evaluation of these terms on the grids are demanding, and very dependent on the functional. | ||
+ | |||
+ | < | ||
+ | &XC | ||
+ | & | ||
+ | &END XC_FUNCTIONAL | ||
+ | & | ||
+ | ! defaults | ||
+ | XC_SMOOTH_RHO NONE | ||
+ | XC_DERIV PW | ||
+ | &END XC_GRID | ||
+ | &END XC | ||
+ | </ | ||
+ | |||
+ | For BLYP functional some smoothing needs to be applied. The smoothing may also converge forces more rapidly than the default settings, but at the expense of modifying the functional slightly. | ||
+ | |||
+ | < | ||
+ | |||
+ | compare to the previous calculation, | ||
+ | |||
+ | < | ||
+ | &XC | ||
+ | & | ||
+ | &END XC_FUNCTIONAL | ||
+ | & | ||
+ | XC_SMOOTH_RHO NN50 | ||
+ | XC_DERIV NN50_SMOOTH | ||
+ | &END | ||
+ | &END XC | ||
+ | </ | ||
+ | | ||
+ | compare the convergence of LDA and BLYP to PBE. | ||
+ | |||
+ | < | ||
+ | & | ||
+ | &END XC_FUNCTIONAL | ||
+ | </ | ||
+ | </ | ||
+ | |||
+ | <note tip> | ||
+ | Also change the psuedo potential to the appropriate functional. | ||
+ | < | ||
+ | &KIND O | ||
+ | BASIS_SET DZVP-MOLOPT-SR-GTH-q6 | ||
+ | POTENTIAL GTH-PADE-q6 | ||
+ | &END KIND | ||
+ | </ | ||
+ | PADE is a synonym for LDA. | ||
+ | </ | ||
+ |
events/2018_summer_school/converging_cutoff.txt · Last modified: 2020/08/21 10:15 by 127.0.0.1