  The long-wavelength coherent overdensity and tidal force, which are not
direct observables for a finite-volume survey, affect time evolution of cosmic
structure formation and therefore clustering observables through the mode
coupling. In this paper we develop an "anisotropic" separate universe (SU)
simulation technique to simulate large-scale structure formation taking into
account the effect of large-scale tidal force into the anisotropic expansion of
local background. We modify the TreePM N-body simulation code to implement the
anisotropic SU simulations, and then study the "response" function of matter
power spectrum that describes how the matter power spectrum responds to the
large-scale tidal effect as a function of wavenumber and redshift for a given
global cosmology. We test and validate the SU simulation results from the
comparison with the perturbation theory predictions and the results from
high-resolution PM simulations. We find that the response function displays
characteristic scale dependences over the range of scales down to nonlinear
scales, up to k ~ 6 h/Mpc.
