PULSUS: Direct Frequency-Domain Multidimensional Spectroscopy Beyond the Impulsive Rotating-Wave Approximation
We introduce PULSUS, a direct frequency-domain framework for multidimensional spectroscopy of open quantum systems beyond the impulsive rotating-wave approximation. Coherence-frequency intervals are represented through Liouvillian resolvents, while finite Gaussian pulses are incorporated through operator-valued functions of the field-free Liouvillian. This enables direct evaluation on Cartesian grids or arbitrary spectral coordinates while retaining molecular evolution during the pulse envelopes and, when required, the complete light--matter interaction. The implementation is validated against analytical and independent time-domain benchmarks and supports targeted spectral sampling that reduces the number of evaluated coordinates when the response is spectrally localized. A controlled hierarchy of finite-pulse, rotating-wave, short-pulse, and impulsive models separates the errors introduced by each approximation. For a coupled electronic dimer, the total impulsive-RWA error is nonmonotonic in pulse duration: an intermediate regime provides the highest accuracy, whereas ultrashort pulses expose coupling-enabled beyond-RWA pathways and longer pulses develop substantial finite-pulse corrections. These results show that the impulsive and rotating-wave approximations cannot be assumed to improve simultaneously and should be tested when quantitatively interpreting multidimensional spectra. PULSUS provides a direct frequency-domain framework for carrying out that comparison.