Hi everyone,

I am trying to launch some anharmonic calculations on methane and its tritium derivatives, but I am facing some problem: the calculated anharmonic frequency are pretty far from the experimental one and moreover I got some warning in the VPT2 part.

I put here my input file at B3LYP/def2TZVPPD level

%Mem=10GB

%NProc=64

%Chk=BTP.chk

# b3lyp/gen opt freq(raman,anharm) int=ultrafine empiricaldispersion=GD3BJ Nosymm Pop=Full IOP(6/7=3,9/40=45)

Title Card Required

0 1

C 2.79026200 0.06285100 0.79569900

H 3.15295600 -0.96304100 0.79569900

H 3.15297500 0.57578900 1.68414300

H 3.15297500 0.57578900 -0.09274600

H 1.70214500 0.06286400 0.79569900

H 0

S 3 1.00

34.0613410 0.60251978D-02

5.1235746 0.45021094D-01

1.1646626 0.20189726

S 1 1.00

0.32723041 1.0000000

S 1 1.00

0.10307241 1.0000000

P 1 1.00

1.40700000 1.0000000

P 1 1.00

0.38800000 1.0000000

P 1 1.00

0.95774129632D-01 1.0000000

D 1 1.00

1.05700000 1.0000000

****

C 0

S 6 1.00

13575.3496820 0.22245814352D-03

2035.2333680 0.17232738252D-02

463.22562359 0.89255715314D-02

131.20019598 0.35727984502D-01

42.853015891 0.11076259931

15.584185766 0.24295627626

S 2 1.00

6.2067138508 0.41440263448

2.5764896527 0.23744968655

S 1 1.00

0.57696339419 1.0000000

S 1 1.00

0.22972831358 1.0000000

S 1 1.00

0.95164440028D-01 1.0000000

S 1 1.00

0.48475401370D-01 1.0000000

P 4 1.00

34.697232244 0.53333657805D-02

7.9582622826 0.35864109092D-01

2.3780826883 0.14215873329

0.81433208183 0.34270471845

P 1 1.00

0.28887547253 .46445822433

P 1 1.00

0.10056823671 .24955789874

D 1 1.00

1.09700000 1.0000000

D 1 1.00

0.31800000 1.0000000

D 1 1.00

0.90985336424D-01 1.0000000

F 1 1.00

0.76100000 1.0000000

****

and the Warnings are

Data for Anharmonic Potential Energy Surface

--------------------------------------------

WARNING: Unreliable CUBIC force constant i= 3,j= 4,k= 4

- Fjik = 0.12890637 cm^-1 while Fijk is NULL

WARNING: Unreliable CUBIC force constant i= 2,j= 3,k= 4

- Fjik = -0.07129306 cm^-1 while MEAN(Fijk,Fjik) is NULL

Meanwhile the calculated frequency are reported below, which are far from the experimental one:

Anharmonic Infrared Spectroscopy

==================================================

Units: Transition energies (E) in cm^-1

Integrated intensity (I) in km.mol^-1

Fundamental Bands

-----------------

Mode(n) E(harm) E(anharm) I(harm) I(anharm)

1(1) 3136.992 2986.364 22.11749694 24.16422094

2(1) 3136.982 3003.557 22.11750213 24.34210596

3(1) 3136.976 3006.676 22.11760596 24.36770242

4(1) 3033.399 2908.586 0.00000014 0.00004897

5(1) 1557.978 1522.322 0.00000000 0.00000071

6(1) 1557.973 1522.376 0.00000001 0.00000032

7(1) 1340.477 1308.298 11.55465076 11.57586740

8(1) 1340.460 1306.912 11.55454570 11.55789092

9(1) 1340.458 1309.397 11.55467054 11.59354191

Do you know how I could solve this issues? Additionally I wanted to ask why I see just few modes and not 3*N-6 modes ( the low frequency modes are just 6 and I don't know ho to look at the degeneracy of the vibrations from the output).

Thanking you in advance

Francesco