[CP2K-user] Kinetic Energy from Velocities

[fei]

Hi, to make things easier in demonstrating what Claudio is describing I put 
a Li atom in a box and requested that its initial temperature be 1200K. 

 

CP2k writes:

 

#     Step Nr.          Time[fs]        Kin.[a.u.]          
Temp[K]            Pot.[a.u.]        Cons Qty[a.u.]        UsedTime[s]

         0            0.000000         *0.005700268 *     
1200.000000000         0.000000000         0.011400535         0.000000000

 

The velocity reported for that single atoms is:

       1

 i =        0, time =        0.000, E =         0.0000000000

 Li         *0.0000000000        0.0000000000        0.0009492302*

 

For that single atom we have:


0.5*m*v^2=0.5*6.914*0.0009492302^2=*0.00000312705*  This is in CP2K's 
atomic units.



Now if I follow the manual’s instruction “Having an atom with a mass m in 
AMU the kinetic energy 1/2mv^2 will be obtained in Hartree (i.e. au) 
multiplying by 911.447”

I get:


0.00000312705*911.447=*0.00285014242* 

 

which is exactly half of the kinetic energy reported by CP2K.


[0.005700268 /  2]  see beginning of this same post.

Could someone kindly clarify how this is possible/correct?

Also, the reported temperature matches the reported kinetic energy but 
obviously not what one would derive from the velocity.


Thank you,

Fei



On Tuesday, June 18, 2019 at 7:50:04 PM UTC-5, Claudio wrote:

> Dear all, 
>
> first thanks in advance for your help.
>
> I have a very simple question (perhaps a units misunderstanding on my 
> part). I am simulating classically in the NVT ensemble a simple AB type 
> molten salt with a polarizable potential. When I write out the velocities 
> along my simulation and I try to compute the kinetic energy using these via 
> the expression:
>
> 1/2 Sum_i { m_i v_i dot v_i} and multiply by the conversion 
> factor  911.447 --as per the manual-- 
>
> "Having an atom with a mass m in AMU the kinetic energy 1/2mv^2 will be 
> obtained in Hartree (i.e. au) multiplying by 911.447"
>
> I get a number in Hartrees that is exactly 1/2 of that printed out by CP2K 
> each frame. I can't seem to understand how to reconcile these two numbers. 
> Is there something I am overlooking?
>
> Thanks once again for your kind help and consideration. I can provide the 
> velocities of a snapshot and a simple code to compute the kinetic energy if 
> needed, but this is so simple that I doubt the problem is in the code.
>
> Best regards
>
> Claudio
>
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