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428 chapter 16the molecules stay close to each other, the polarities continue to fluctuate in synchronization⇐⇐ so that the attraction is maintained. The resultant dipole–dipoleattraction behaves like 1/r 6 , and although much weaker than a Coulomb force, itis responsible for the binding of neutral, inert elements, such as argon for whichthe Coulomb force vanishes.16.1.1 Connection to Thermodynamic VariablesWe assume that the number of particles is large enough to use statistical mechanicsto relate the results of our simulation to the thermodynamic quantities (the simulationis valid for any number of particles, but the use of statistics requires largenumbers). The equipartition theorem tells us that for molecules in thermal equilibriumat temperature T , each molecular degree of freedom has an energy k B T/2on the average associated with it, where k B =1.38 × 10 −23 J/K is Boltzmann’sconstant. A simulation provides the kinetic energy of translation 4 :KE = 1 2〈 N−1 ∑i=0v 2 i〉. (16.9)The time average of KE (three degrees of freedom) is related to temperature by〈KE〉 = N 3 2 k BT⇒ T = 2〈KE〉3k B N . (16.10)The system’s pressure P is determined by a version of the Virial theorem,PV = Nk B T + w 3 ,〈 N−1〉∑ w= r ij · f ij , (16.11)i