[molpro-user] cp-corr. Potential DF-LCCSD(T0), numerical accuracy

Berger Raphael berger at chem.helsinki.fi
Wed Jan 21 06:34:06 GMT 2009

Thank You Gerald and Grant,

in my case {hf,accu,16} did the trick.

Because i have a small system with 
all strongly interacting fragments and i only wanted to scan a _tiny_ 
region on the PES. So domains did'nt change also just with the defaults, 
and  since the fragments interact strongly i do not want to use the 
local-treatment to correct for bsse. My goal is to get only some 
estimation about the general size of the intermolecular BSSE in terms of 
eq. geo and total energy in this 3-atomic system.
I wanted still to use LCCSD because it should be a kind of benchmark for 
the methods performance when I then go for the bigger molecule.

Thank You for advice
 	R. Berger

On Tue, 20 Jan 2009, Gerald Knizia wrote:

> On Tuesday 20 January 2009 16:53, Berger Raphael wrote:
>> when trying to calculate a cp-corrected potential curve
>> in a close region around a uncorrected minimum
>> it seems to give a "noisy" curve for the dE_cp using
>> only the default settings. The cp-correction at the
>> equlibrium geometry is about 8*10^-4 au.
>> The level is DF-LCCSD(T0)/SARC-TZVPP with DKH3
>> integrals.
> Local CCSD works by neglecting parts of the wave function with small impact.
> Which parts fall below the thresholds depends on the molecular geometry. This
> leads to non-continuous potential energy surfaces unless special care is
> taken about the domains and pairs (lookup mergedom in the manual).
> This does not happen if you use conventional CCSD(T), which also usually is
> faster for small calculations.
> -- 
> Gerald Knizia
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