STATIC RESPONSE FROM QUANTUM MONTE-CARLO CALCULATIONS

We have evaluated the density-density static response of a many-body system by calculating with the quantum Monte Carlo method the energy and density change caused by an external potential. Our results for the linear response function of liquid He-4 at zero pressure and temperature are in excellent agreement with the available experimental data. The results for the response function of 2D electrons also at zero temperature, obtained within the fixed-node approximation, constitute the most accurate information available to date for this system.