2 LOW-TEMPERATURE APPLICATIONS OF PATH-INTEGRALS - SUPERFLUIDITY IN HE-4 AND ORIENTATIONAL ORDERING IN SOLID H2, D2

Computational path integrals, because of the intuitive picture they pr ovide and their natural inclusion of temperature, are being increasing ly used to simulate low temperature systems. We discuss two such appli cations. (1) The superfluid transition of He-4 in three dimensions, wh ere path integrals combined with finite-size scaling techniques can gi ve transition temperatures accurate to within 1 or 2%. (2) Orientation ordering in solid H-2, D2 where, using a fixed node approximation, th e ordering temperature for ortho H-2 can be reproduced to within about 15% in contrast to mean-field-type calculations which predict transit ion temperatures too high by factors of two to three.