Simen Kvaal

Elias Jarlebring, Simen Kvaal, Wim Michiels

Consider a symmetric matrix $A(v)\in\RR^{n\times n}$ depending on a vector $v\in\RR^n$ and satisfying the property $A(αv)=A(v)$ for any $α\in\RR\backslash{0}$. We will here study the problem of finding $(λ,v)\in\RR\times \RR^n\backslash\{0\}$ such that $(λ,v)$ is an eigenpair of the matrix $A(v)$ and we propose a generalization of inverse iteration for eigenvalue problems with this type of eigenvector nonlinearity. The convergence of the proposed method is studied and several convergence properties are shown to be analogous to inverse iteration for standard eigenvalue problems, including local convergence properties. The algorithm is also shown to be equivalent to a particular discretization of an associated ordinary differential equation, if the shift is chosen in a particular way. The algorithm is adapted to a variant of the Schrödinger equation known as the Gross-Pitaevskii equation. We use numerical simulations toillustrate the convergence properties, as well as the efficiency of the algorithm and the adaption.

Simen Kvaal

Absorbing boundary conditions in the form of a complex absorbing potential are routinely introduced in the Schrödinger equation to limit the computational domain or to study reactive scattering events using the multi-configurational time-dependent Hartree method (MCTDH). However, it is known that a pure wave-function description does not allow the modeling and propagation of the remnants of a system of which some parts are removed by the absorbing boundary. It was recently shown [S. Selstø and S. Kvaal, J. Phys. B: At. Mol. Opt. Phys. {\bfseries 43} (2010), 065004] that a master equation of Lindblad form was necessary for such a description. We formulate a multiconfigurational time-dependent Hartree method for this master equation, usable for any quantum system composed of any mixture of species. The formulation is a strict generalization of pure-state propagation using standard MCTDH. We demonstrate the formulation with a numerical experiment.

Andre Laestadius, Simen Kvaal

The mathematical foundation of the so-called extended coupled-cluster method for the solution of the many-fermion Schrödinger equation is here developed. We prove an existence and uniqueness result, both in the full infinite-dimensional amplitude space as well as for discretized versions of it. The extended coupled-cluster method is formulated as a critical point of an energy function using a generalization of the Rayleigh-Ritz principle: the bivariational principle. This gives a quadratic bound for the energy error in the discretized case. The existence and uniqueness results are proved using a type of monotonicity property for the flipped gradient of the energy function. Comparisons to the analysis of the standard coupled-cluster method is made, and it is argued that the bivariational principle is a useful tool, both for studying coupled-cluster type methods, and for developing new computational schemes in general.

Simen Kvaal

A dynamical formulation of coupled cluster theory is derived using a variational principle. By allowing time-dependent single-particle functions, a high degree of adaptivity is introduced, allowing complex systems to be simulated with high accuracy. Equations of motion are derived which are shown to be suitable for computer implementation. The method, called adaptive time-dependent coupled cluster, is a strict generalization of the formulation used in standard coupled cluster response theory, and also represents a systematic hierarchy of size-consistent approximations, including standard time-dependent Hartree--Fock as a trivial case.