Accurate determination of the critical line of the square Ising antiferromagnet in a field

Several points on the critical line of the simple quadratic antiferromagnetic Ising model in a field are located with high precision. This is accomplished by means of finite-size scaling of the magnetic correlation length of n* infinity strips. The latter quantities were determined by means of the transfer matrix method for strip widths up to n=20. An expression, possibly with the exact form, is constructed which reproduces the critical line with an accuracy to the order of 10 -10 .