TURBOMOLE Users Forum
TURBOMOLE Modules => Jobex: Structure Optimization and Molecular Dynamics => Topic started by: wilhelm on June 09, 2010, 10:29:59 AM
-
Hi,
currently I am trying to reproduce silane geometries calculated at the CCSD(t)/aug-cc-pV(X+d)Z level of theory with Molpro and Gaussian using the CC2 approximation in TURBOMOLE. Unfortunately, the bond lengths are about 0,02 A too short and e. g. B3LYP gives far better bond lengths (angles are ok btw.).
I am a bit confused about the big failure and would like to know if this is a known issue or if I am doing something wrong.
control file: (SiH4 with aug-cc-pV(5+d)Z)
$title
$operating system unix
$symmetry c1
$redundant file=coord
$coord file=coord
$user-defined bonds file=coord
$atoms
si 1 \
basis =si aug-cc-pV(5+d)Z \
cbas =si aug-cc-pV(5+d)Z
h 2-5 \
basis =h aug-cc-pV5Z \
cbas =h aug-cc-pV5Z
$basis file=basis
$rundimensions
dim(fock,dens)=190284
natoms=5
nshell=110
nbf(CAO)=612
nbf(AO)=456
dim(trafo[SAO<-->AO/CAO])=945
rhfshells=1
nt1amt=4023
$scfmo file=mos
$closed shells
a 1-9 ( 2 )
$scfconv 8
$thize 0.10000000E-04
$thime 5
$scfdamp start=0.300 step=0.050 min=0.100
$scfdump
$scfintunit
unit=30 size=0 file=twoint
$scfdiis
$scforbitalshift automatic=.1
$drvopt
cartesian on
basis off
global off
hessian on
dipole on
nuclear polarizability
$interconversion off
qconv=1.d-7
maxiter=25
$optimize
internal on
redundant on
cartesian off
global off
basis off logarithm
$coordinateupdate
dqmax=0.3
interpolate on
statistics 5
$forceupdate
ahlrichs numgeo=0 mingeo=3 maxgeo=4 modus=<g|dq> dynamic fail=0.3
threig=0.005 reseig=0.005 thrbig=3.0 scale=1.00 damping=0.0
$forceinit on
diag=default
$energy file=energy
$grad file=gradient
$forceapprox file=forceapprox
$lock off
$maxcor 240
$denconv 0.10000000E-06
$cbas file=auxbasis
$ricc2
cc2
geoopt model=cc2 state=(a 1)
$actual step force
$scfiterlimit=200
$last SCF energy change = -.18021069E-07
$dipole from ricc2
x 0.00001349666390 y 0.00001236855594 z -0.00004665868877 a.u.
| dipole | = 0.0001273974 debye
$last CC2 energy change= 0.92946462E-08
$optinfo file=optinfo
$hessapprox file=hessapprox
$forceconv 7
$statpt
itrvec 0
$orbital_max_rnorm 0.14520326152702E-04
$end
Thanks,
Wilhelm
-
Well, CC2 is not CCSD(T)... so clearly you cann't reproduce CCSD(T) bond lengths with CC2.
To more clear: the CC2 method has been develop for calculations on excited states and was never intended for ground state calculations (see the user manual and the references given therein or a good quantum chemistry textbook).
For ground state geometries you should use B3LYP or MP2. They are much more appropriate for this purpose.
Regards,
Christof
-
Hi,
Perhaps this is not directly related to the original post, but I think it could be a good thing to be more clear between CC2 and MP2 for ground state calculations (e.g. energies and geometries). I just followed a presentation where after optimizing the geometry with DFT, the "accurate" ground state energies were obtained using CC2. After asking why they prefer CC2 over MP2 for their energies, the answer was that it's a higher order method and therefore more accurate.
Christof's message is quite clear but I took a look at the RICC2 section of the manual but didn't find a warning that you should not use CC2 for ground state energies. Is it just that you spend more CPU time or do the results (ground state energies/geometries) get worse and, if so, by how much?
regards,
Olli
-
Dear Olli,
There is no warning in the manual, since we assume that users first check (based on text books or some reference literature) which method they should apply for their problems. A program manual is not the right place to learn the basics of quantum chemistry and which method should be applied to which problem.
Anyway, you can read about the accuracy of CC2 for ground state structures and harmonic frequencies in J. Chem. Phys. 118, 7751-7761 (2003).
Best wishes,
Christof