CC2 optimizations and bond lengths

[wilhelm]

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

[christof.haettig]

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

[olehtone]

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

[christof.haettig]

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