The importance of having accurate second virial coefficients in phase equilibrium calculations, especially for the calculation of dew points, is discussed. The square-well potential results in a simple but inaccurate equation for the second virial coefficient for small, spherical molecules such as argon. Here, we present a new equation for the second virial coefficient of both spherical molecules and chain molecules which is written in a form similar to that for the square-well potential. This new equation is accurate in comparison to Monte Carlo simulation data on second virial coefficients for square-well chain molecules and with second virial coefficients obtained from experiments on n-alkanes.