How to obtain accurate frequencies from CP2K? (inside the post the frequencies are compared between CP2K and Gaussian)

[Matt W]

Hi,

you are not consistently running a non-periodic system - you need a 
nonperiodic poisson section too. And 

1. You have 

&VIBRATIONAL_ANALYSIS
    DX 1.0E-03
    NPROC_REP  24
    FULLY_PERIODIC TRUE

the fully periodic keyword means that CP2K won't remove rotations (because 
they aren't there for a crystal with pbc) - so you get an extra 3 modes.

2. You are running a totally different 'model chemistry' in Gaussian 
terminology. You can't compare M062X with PBE directly. PBE (GGAs) 
typically underbind, bonds too long, frequencies too small. M06 maybe is 
better on average.

3 So you are mixing two things - accurate, i.e. properly performed 
calculation at a given level of theory - and agreement with experiment. 
Changing the model will change the frequencies. Changing settings (CUTOFF 
mainly, probably) will just ensure you get the correct numbers for the 
model. You should run PBE in Gaussian to compare to, if this is what you 
want.

Matt

On Wednesday, November 23, 2016 at 9:40:31 PM UTC, J. Ye wrote:
>
> Hi all, 
>
> I calculate frequency for small molecules and try to compare with Gaussian 
> 09 and experimental results. I show the results from *CP2K *(PBE 
> functional-BASIS_MOPLOT, GTH potental, nonperiodic) and * Gaussian09 *(M062X/6-311g(d,p)) 
> . 
>
> I have several questions:
>
> 1. You will see the total number of modes obtained from CP2K is 15, 
> however 12 modes from Gaussian. Are the first three modes are translation 
> modes or rotation modes for C2H4?
> 2. If we only compare the 12 vibration modes, the value for most of  modes 
> from Gaussian is larger than CP2K. What's the reason for getting smaller 
> frequencies from CP2K?  
> 3. What is the keyword can strongly affect the frequencies?  Affect the 
> accuracy of frequencies? 
>
> Thanks very much! 
>
> CP2K Gaussian 
> c2h4 
> 15 modes  
> -19.0703 
> -5.64472 
> 19.50384 
> 807.9613 831.8134 
> 942.954 988.1607 
> 995.6397 996.8044 
> 1045.988 1071.741 
> 1206.11 1239.784 
> 1373.488 1386.642 
> 1446.925 1470.955 
> 1646.778 1717.766 
> 3158.382 3156.212 
> 3171.654 3172.992 
> 3269.88 3235.274 
> 3270.754 3260.871 
>
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