Nonlinear response theory for time-periodic elongational flows

The time-dependent nonlinear response theory of Petravic and Evans [Phys. Rev. Lett. 78, 1199 (1997); Phys. Rev. E 56, 1207 (1997)] is applied to the case of an atomic fluid undergoing oscillatory elongational flow. It is found that nonlinear response calculations agree very well with direct nonequilibrium molecular dynamics calculations of the diagonal elements of the pressure tensor. At very weak applied fields, nonlinear response theory is statistically superior to the direct calculation, which suffers from low signal to noise ratios, but at higher fields direct averaging of phase variables is observed to be superior. [S1063-651X(98)12210-9].