Hi Psi4 community,
I thought I would ask if second-order vibrational perturbation theory (VPT2) is available in Psi4. Specifically, I would like to compute a polyatomic molecule’s rotational constant. I didn’t see it listed in the manual or webpages.
Thanks,
Karl
So a rotational constant can be got from just the molecule geometry through the moments of inertia. They’re always printed out like so.
On the other hand, one can get more detailed info (below) out of a VPT2 calc in Cfour (http://cfour.de/), to which Psi does have an early interface.
What level of info are you seeking?
----------------------------------------------------------------------------
VIB-ROT CONSTANT / (cm-1)
------------------------------
AXIS MODE CORIOLIS QUADRATIC ANHARMONIC TOTAL
----------------------------------------------------------------------------
1 7 0.2051569 -0.0009611 -0.0415298 0.1626660
1 8 0.0080145 -0.1381353 0.3158351 0.1857143
1 9 -0.1951076 0.0000000 0.3267314 0.1316239
2 7 0.0000000 -1.5950786 -0.4647836 -2.0598623
2 8 0.0000000 -0.3308519 1.1957598 0.8649079
2 9 0.0000000 -0.3788200 1.6437854 1.2649654
3 7 0.0000000 -0.2072040 0.0407846 -0.1664194
3 8 0.0000000 -0.2328957 0.3971091 0.1642134
3 9 0.0000000 -0.2083142 0.2878044 0.0794902
----------------------------------------------------------------------------
----------------------------------------------------------------------------
Be, B0 AND B-B0 SHIFTS FOR SINGLY EXCITED VIBRATIONAL STATES (CM-1)
------------------------------------------------------------
VIBRATION X AXIS Y AXIS Z AXIS
----------------------------------------------------------------------------
Be 9.39809364 26.48855480 14.56613231
B0 9.15809152 26.45354928 14.52749024
Be-B0 0.24000212 0.03500552 0.03864207
B' 9.15526821 26.45543149 14.52937245
Be-B' 0.24282544 0.03312331 0.03675986
B'' 9.15277799 26.45604409 14.53099494
Be-B'' 0.24531565 0.03251070 0.03513737
B^A 9.15227534 26.45413298 14.53366382
Be-B^A 0.24581830 0.03442181 0.03246849
B^S 9.15305855 26.45432458 14.53272734
Be-B^S 0.24503510 0.03423021 0.03340497
7 -0.16266604 2.05986226 0.16641941
8 -0.18571434 -0.86490787 -0.16421336
9 -0.13162385 -1.26496543 -0.07949019
----------------------------------------------------------------------------
Hi Lori,
Thanks for the reply. My goal is to validate a theory level for geometry optimization by comparing computed rotational constants (RCs) to known experimental values. Someone suggested that I use VPT2 (which is new to me) to obtain vibrationally average RCs. I have compared the values given by Psi4 (via the moments of inertia), but I wanted to also try this other way too.
Bests,
Karl
If VPT2 is unfamiliar to you, I should point out that it requires a series of displacements along each vibrational normal coordinate and then a full frequency calculation at each of those displacements. How big is your molecular system?
Right now, my systems are fairly small - the largest being propanol.
Ok, step 1 is to get Cfour. No charge, but you’ll need to get a license (website above) and get it installed (binary should be fine).