A new classification of spin-adapted MRCISD states using the hole-particle correspondence is given and formulas for the hole loop shapes are derived. Based on the classification and the formulas for hole loop shapes, an improved CI algorithm is suggested and coded. The new algorithm reduces disk storage requirement by over one hundred folds for an MRCISD calculation with 37 doubly occupied orbitals. Additionally, the code is very efficient. For a calculation of 10(7) CSFs, the average time per iteration is only 650 s on a PC. (C) 2003 Elsevier Science B.V. All rights reserved.