Regularized numerical methods for the nonlinear Schrödinger equation with singular nonlinearity

We present different regularizations and numerical methods for the nonlinear Schr\"odinger equation with singular nonlinearity (sNLSE) including the regularized Lie-Trotter time-splitting (LTTS) methods and regularized Lawson-type exponential integrator (LTEI) methods. Due to the blowup of the singular nonlinearity, i.e., $f(\rho)=\rho^{\alpha}$ with a fixed exponent $\alpha<0$ goes to infinity when $\rho \to 0^+$ ($\rho = |\psi|^2$ represents the density with $\psi$ being the complex-valued wave function or order parameter), there are significant difficulties in designing accurate and efficient numerical schemes to solve the sNLSE. In order to suppress the round-off error and avoid blowup near $\rho = 0^+$, two types of regularizations for the sNLSE are proposed with a small regularization parameter $0 < \eps \ll 1$. One is based on the local energy regularization (LER) for the sNLSE via regularizing the energy density $F(\rho) = \frac{1}{\alpha+1}\rho^{\alpha+1}$ locally near $\rho = 0^+$ with a polynomial approximation and then obtaining a local energy regularized nonlinear Schr\"odinger equation via energy variation. The other one is the global nonlinearity regularization which directly regularizes the singular nonlinearity $f(\rho)=\rho^{\alpha}$ to avoid blowup near $\rho = 0^+$. For the regularized models, we apply the first-order Lie-Trotter time-splitting method and Lawson-type exponential integrator method for temporal discretization and combine with the Fourier pseudospectral method in space to numerically solve them. Numerical examples are provided to show the convergence of the regularized models to the sNLSE and they suggest that the local energy regularization performs better than directly regularizing the singular nonlinearity globally.