Local correlation methods with linear scaling of the computational cost as function of the molecular size have much extended the applicability of high-level electronic structure methods. Recent developments in our group [1-8] will be reviewed, and the accuracy and efficiency of these methods for computing reaction enthalpies as well as other molecular properties [3] and excitation energies [4] will be demonstrated. The bottleneck in such calculations is often the calculation of the 2-electron integrals and their transformation into the basis of local molecular orbitals (LMOs) and projected atomic orbitals (PAOs), in particular for large basis sets. This problem can be strongly reduced using local density fitting (DF) methods. It will be demonstrated that the CPU-times for integral evaluation and transformation in LMP2 and LCCSD can be reduced by 1-2 orders of magnitude [5,6], virtually without loss of accuracy. Similar improvements are possible for LMP2 gradients [7] and Fock-matrix evaluation [5]. Recently, we have also developed an efficient DF-LMP2-R12 method [8], in which all required integrals are obtained using robust density fitting approximations [9]. This method has been applied for calculations on molecules with up to 90 atoms and over 2500 basis functions.\\

[1] M. Schütz and H.-J. Werner, Chem. Phys. Lett. 318, 370 (2000).

[2] M. Schütz and H.-J. Werner, Chem. Phys. Lett. 114, 661 (2001).

[3] T. Korona, K. Pfl"uger, and H.-J. Werner, Phys. Chem. Chem. Phys., in press.

[4] T. Korona and H.-J. Werner, J. Chem. Phys. 118, 3006 (2003).

[5] H.-J. Werner, F. R. Manby, and P. J. Knowles, J. Chem. Phys., 118, 8149 (2003)

[6] M. Schütz and F. R. Manby, Phys. Chem. Chem. Phys., 118, 3349 (2003)

[7] M. Schütz, H.-J. Werner, R. Lindh, and F. R. Manby, submitted for publication.

[8] F. R. Manby and H.-J. Werner, to be published.

[9] F. R. Manby, J. Chem. Phys. 119, 4607 (2003)