evdwl ecoul

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1

Dear all,

I’m a little confused about the computation of total evdwl and ecoul. The output file results in “pe/mol/tally” and "thermo_style " are different.

In my system ,there are a,b,c and d.

The output of thermo_style evdwl ecoul is 31034.856 and -76830.515

The output of pe/mol/tally is the following:

compute apair a pe/mol/tally a

0 298315 17478.33529 -99421.15882

compute bpair b pe/mol/tally b
1446.858824 19150.07647 -15496.78824 32.20585882

compute cpair c pe/mol/tally c

27.93761176 272.8468824 -372.8372941 -199.7706471

compute dpair d pe/mol/tally d

22174 -247652 255.4768824 -2363.910588
compute allpair all pe/mol/tally all

23648.79412 70086.41765 7358.913529 -146867.5294

In addition, I use formula E=Etotal-(Ea+Eb+Ec+Ed) to calculate the results as follows

0 0 5494.726882 -44914.89521

Why the evdwl and ecoul results calculated by “pe/mol/tally” do not correspond to “thermo_style”?

(From the manual and mail list,

Compute pe/mol/tally calculates a global 4-element vector containing (in this order): evdwl and ecoul for intramolecular pairs and evdwl and ecoul for intermolecular pairs)

Looking forward to your reply. Thank you very much

Sincere regards to you

ZZ

2

they are different, because you are computing different things.

when you are computing the partial tallies, you only compute the sum over pairs of atoms from a group with atoms from the same group.
you are missing the contributions from interactions of group a with group b, group a with group c and so on. those are included in the global sums.

you would get the global sum only with something like this.

compute apair a pe/mol/tally all

compute bpair b pe/mol/tally all

axel.

3

Thank you very much, Axel.

I still have a little question. Here is my understanding of your explanation.

I learned about the interaction energy (Ea−b) Between a molecules and b is defined as Ea−b = Eab − (Ea + Eb) from a literature, where Eab is the total energy of a molecules and b and E****b and Ea are the energies of individual a Molecules and the b aggregate, respectively.

Combined with my understanding of your explanation, it can be calculated as follows.
compute apair a pe/mol/tally all
compute bpair b pe/mol/tally all

compute abpair ab pe/mol/tally all

Ea−b=abpair - (apair + bpair)

The above is my understanding of your explanation, I don’t know whether it is accurate or no.

Thank you very much