[gmx-users] How to calculate the energy drift?

[xuji]

Hi all Gromacs users:

In Gromacs 4 paper
"GROMACS 4 Algorithms for Highly Efficient,Load-Balanced, and Scalable Molecular Simulation"
"The energy drift for GROMACS is 0.01 kBT/ns per degree of freedom."

But I don't know how to calculate the value of energy drift. Is this energy the kinetic energy or
the sum of kinetic and potential energies? 

And if the simulation runs 20ns, is the energy drift the value of energy difference 
between first and last step divided by the simulation time (20ns)?

Appreciate for any help in advance!
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