From NWChem
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Posts 86


10:22:34 PM PDT  Fri, May 3rd 2013 

I run RIMP2 calculation of cubane molecule (C8H8) in a different bases.
ccpCVTZ, ccpCVQZ looks good, but with ccpCV5Z basis I have got unrelailable MP2 energy.
my input file is (molecular geometry is almost equilibrium):
title "cubane ccpV5Z MP2RI energy"
geometry
symmetry Oh
H 1.41063149 1.41063149 1.41063149
C 0.78371973 0.78371973 0.78371973
end
basis
H library ccpV5Z
C library ccpCV5Z
end
basis "rimp2 basis"
H library ccpV5Zri
C library ccpV5Zri
end
task rimp2
output energy is:

SCF energy 307.507255162945
Correlation energy 4369.270156917300
Total MP2 energy 4676.777412080244


Edited On 10:24:52 PM PDT  Fri, May 3rd 2013 by Vladimir




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6:54:42 AM PDT  Mon, May 6th 2013 

Use of spherical functions can do the trick!
title "cubane ccpV5Z MP2RI energy"
geometry
symmetry Oh
H 1.41063149 1.41063149 1.41063149
C 0.78371973 0.78371973 0.78371973
end
basis spherical
H library ccpV5Z
C library ccpCV5Z
end
basis "rimp2 basis"
H library ccpV5Zri
C library ccpV5Zri
end
scf
direct
end
task rimp2
output energy is:

SCF energy 307.507287526028
Correlation energy 1.815754093360
Total MP2 energy 309.323041619388

many thanx to Endlesslake

Edited On 8:40:43 AM PDT  Mon, May 6th 2013 by Vladimir




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12:15:58 AM PDT  Fri, Jun 7th 2013 

Dunning basis sets are designed to be used with spherical functions so the cartesian functions cause very bad linear dependency problems in some cases. Spherical functions aren't a trick; they are the right way to run this calculation.



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