Algorithm Development In Density Matrix Renormalization Group And Canonical Transformation Theory

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In this thesis, we will describe several extensions of the Density Matrix Renormalization Group (DMRG) and Canonical Transformation Theory (CT). In the first part we describe a new way to solve for excited states in DMRG. To overcome the limitations of the traditional state-averaging approaches in excited state calculations, where one solves for all states between the ground state and excited state of interest, we have investigated a number of new excited state algorithms. Building on the work of van der Vorst and Sleijpen (SIAM J. Matrix Anal. Appl., 17, 401 (1996)), we have implemented Harmonic Davidson and State-Averaged Harmonic Davidson algorithms within the context of DMRG. We have assessed their accuracy and stability of convergence in complete active space DMRG calculations on the low-lying excited states in the acenes ranging from naphthalene to pentacene. We find that both algorithms offer increased accuracy over the traditional State-Averaged Davidson approach, and in particular, the State-Averaged Harmonic Davidson algorithm offers an optimal combination of accuracy and stability in convergence. In the second part, we propose an analytic response theory for DMRG whereby response properties correspond to analytic derivatives of DMRG observables with respect to the applied perturbations. Both static and frequencydependent response theories are formulated and implemented. We evaluate our pilot implementation by calculating static and frequency-dependent polar- isabilities of short oligo-di-acetylenes. The analytic response theory is competitive with dynamical DMRG methods and yields significantly improved accuracies when using a small number of density matrix renormalisation group states. Strengths and weaknesses of the analytic approach are discussed. In the third part, we describe how to calculate density matrices in CT theory. Density matrices are useful not only for computing observables but also for characterizing the nature of individual states. We demonstrate this with a preliminary application of the CT theory to understand the low-lying excited states of oligo-phenylvinylenes. We finish by presenting the theory and equations for calculating the response (e.g. to an external field) in the CT and DMRG-CT theories.
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