An extension of the formulation of the atomic-orbital-based response theory of Larsen et al., JCP 113, 8909 (2000) is presented. This new framework has been implemented in LSDalton and allows for the use of Kohn-Sham density-functional theory with approximate treatment of the Coulomb and Exchange contributions to the response equations via the popular resolution-of-the-identity approximation as well as the auxiliary-density matrix method (ADMM). We present benchmark calculations of ground-state energies as well as the linear and quadratic response properties: vertical excitation energies, polarizabilities, and hyperpolarizabilities. The quality of these approximations in a range of basis sets is assessed against reference calculations in a large aug-pcseg-4 basis. Our results confirm that density fitting of the Coulomb contribution can be used without hesitation for all the studied properties. The ADMM treatment of exchange is shown to yield high accuracy for ground-state and excitation energies, whereas for polarizabilities and hyperpolarizabilities the performance gain comes at a cost of accuracy. Excitation energies of a tetrameric model consisting of units of the P700 special pigment of photosystem I have been studied to demonstrate the applicability of the code for a large system. K E Y W O R D S ADMM, density fitting, Kohn-Sham DFT response theory, RI
| I N TR ODU C TI ONIn molecular electronic-structure theory, an essential step is the evaluation of two-electron integrals over one-electron basis functions. The explicit evaluation of these integrals comes at a high computational cost, and from the dawn of quantum chemistry, approximations have been introduced both to speed up molecular calculations and to reduce memory requirements. [1] Such approximate methods have been widely developed for the calculation of energies and gradients, but less attention has been given to developing these methods for the calculation of molecular properties.The most widely used approach to approximate the Coulomb and exchange integrals is density fitting, also known as the resolution-of-theidentity (RI) approximation. In this approximation products of two one-electron basis functions are expanded in one-center auxiliary functions, and thus, the evaluation of four-center two-electron integrals is replaced by the evaluation of two-and three-center two-electron integrals and the solution of a set of linear equations. RI significantly improves performance with a limited impact on the accuracy and has therefore been applied to Hartree-Fock (HF)/Kohn-Sham (KS) theory, as well as correlated methods. [22][23][24][25][26][27] An important alternative approach is the Cholesky-decomposition (CD) technique [28][29][30][31][32] which to a large extent can be thought of as a special kind of density fitting where the auxiliary basis functions are obtained from the set of products between two one-electron basis functions through Cholesky-decomposition.Combined with J-engine techniques [33][34][35] RI gives tremendous speed-ups [8,9] for C...