problem in computing MSD of adatom at the surface

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1

Dear Sir’s
I want to calculate the mean square displacement MSD of Cu adatom moving at Ag (111) surface.
for this purpose i am using command
compute 1 adatom msd
and taking the output using c_1[4]. (adatom is the name of group containg the adatom in the above)

My problem is that, according to theory and previous work MSD value should be increased gradually with time but in my case when adatom moves away from the starting position MSD value increase but when adatom starts moving back towards the starting position MSD value starts decreasing.

I also tried to calculate avearge values but the resultant Avearge MSD is not gruadualy increasing, kindly help me in this regards any help or suggestion will be highly appreciated

Thanks & Regards
Muhammad Imran

2

Dear Sir's
              I want to calculate the mean square displacement MSD of Cu
adatom moving at Ag (111) surface.
for this purpose i am using command
compute 1 adatom msd
and taking the output using c_1[4]. (adatom is the name of group containg
the adatom in the above)
My problem is that, according to theory and previous work MSD value should

what theory? for what kind of system?

axel.