# [CP2K:8433] Re: How to obtain accurate frequencies from CP2K? (inside the post the frequencies are compared between CP2K and Gaussian)

J. Ye jingyu... at gmail.com
Sat Nov 26 22:45:50 CET 2016

```Hi S Ling,

Thanks very much!

How to choose  a correct or reasonable step size (DX in CP2K)? I think DX
value should be as close as the displacement of atoms. However, the
displacement of atoms in real condition should be different. In the
calculation, we are forced to use the same displacement (DX) to calculate
the force constant. How does this affect the frequencies. Just curious?
Maybe it is very minor.

Why there's many other output generate other than f.out?
For example, I have f.inp, f.out and other output files (xxr-1.out,
xx-r-2.out, xx-r-3.out, .......xx-r-9.out). Each xx-r-x.out file looks like
a single point calculations.

Best,

On Thursday, November 24, 2016 at 4:59:44 AM UTC-6, S Ling wrote:
>
> Hi
>
> In addition to what Matt said, if you really want to compare results from
> CP2K and Gaussian, you should: (1) use the all-electron method, i.e. GAPW,
> in CP2K; (2) use the same basis sets; (3) use the same method to calculate
> frequencies. For (3), Gaussian calculates frequencies analytically by
> default, while CP2K calculates frequencies numerically. To calculate
> frequencies numerically in Gaussian, you should specify Freq=Numer. And
> also the calculated frequencies may be sensitive to the the step size for
> numerical differentiation, therefore, you should use the same step size in
> both calculations. In Gaussian, this is controlled by the Step keyword, see
> http://www.gaussian.com/g_tech/g_ur/k_freq.htm; in CP2K, this is
> controlled by the DX keyword.
>
> SL
>
>
>
> On 24 November 2016 at 10:05, Matt W <Matt... at gmail.com> wrote:
>
>> 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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