[CPMD-list] Restart Option for Wave Function & Geometry Optimizatio (Shashidhar)

[Axel Kohlmeyer]

On Wed, 25 Apr 2007, Shashidhar G wrote:

SG> Dear Ari P Seitsonen , Axel Kohlmeyer

dear shashidhar,

SG> Thanks for your quick response and good suggestions and pointing my
SG> mistakes.
SG> 
SG> I did these calculation for Si, Si8, Si18, Si64, Si128, Si256. Up to Si128
SG> in single run I achieved self consistence criterion for wave function
SG> optimization and Idid Geometry optimization and Kohn Sham energy
SG> calculation. But when I came across Si256  I couldn't.

so it would be interesting to know whether your calculations actually
produced results that made any physical sense? being able to complete
the calculation is a required but not sufficient criterion. 

SG> Before doing CPMD , I worked in PARSEC. With PARSEC I went upto Si128, but
SG> after it become difficult with the code.
SG> In both Case I used same *Troullier*-*Martins pseudopotential , *LMAX=P
SG> LOC=S & same cutoff radius.

please explain why you chose that combination. if you read through the
papers describing pseudopotentials, the general recommendation is that
using a higher angular momentum potential is always prefereable as a
local potential. one usually only resolves to lower angular momentum
potentials as local potential if the other choices produce ghost states.

SG> In these two codes for Si system I got approximately equal eigenvalues
SG> values. So I thought my different options in the Input file of CPMD, what
SG> ever I mentioned in my first mail( to cpmd mailing list) is correct.
SG> But after pointing my mistakes, I'm going through your suggestions.
SG> 
SG> *I didn't get  Why we have to choose  LMAX=D (implying LOC=D) is the
SG> standard choice*
SG> *even Si doesn't contain d electrons. Please give me reason or any
SG> reference. *

Si does not have any d-electrons but the availability of empty d-states
is what makes the major difference between the chemistry of silicon and
carbon. many structures containing silicon are only possible by having
d-orbitals participating in bonds. the same is true for S vs. O, P vs. N
and Cl vs. F. for confirmation, please have a look into any regular 
(inorganic) chemistry textbook.

bulk silicon in diamond structure is a bit of an exception, 
since you can usually get away without including the d-channel,
but i am pretty certain that there is a number of papers 
claiming that including d matters.

SG>     |  Full Potential Total Energy =     -288.820070           |
SG>     |  Trouiller-Martins normconserving PP                     |
SG>     |     n    l        rc       energy                        |
SG>     |     3    S    2.5000     -0.40000                        |
SG>     |     3    P    2.5000     -0.15320                        |
SG>     |     3    D    2.5000     -0.15320                        |

FYI: the standard si potentials in the cpmd library have 
rc=1.9 for S and rc=2.1 for p/d ...

cheers,
   axel.

-- 
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Axel Kohlmeyer   akohlmey at cmm.chem.upenn.edu   http://www.cmm.upenn.edu
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