The Nuclear Ensemble Approach (NEA) is a general method for simulations of diverse types of molecular spectra. It works by sampling an ensemble of molecular conformations (nuclear geometries) in the source state, computing the transition probabilities to the target states for each of these geometries, and performing a sum over all these transitions convoluted with shape function. The result is an incoherent spectrum containing absolute band shapes through inhomogeneous broadening. Motivation Spectrum simulation is one of the most fundamental tasks in quantum chemistry. It allows comparing the theoretical results to experimental measurements. There are many theoretical methods for simulating spectra. Some are simple approximations (like stick spectra); others are high-level, accurate approximations (like those based on Fourier-transform of wavepacket propagations). The NEA lies in between. On the one hand, it is intuitive and straightforward to apply, providing much improved results compared to the stick spectrum. On the other hand, it does not recover all spectral effects and delivers a limited spectral resolution. Historical The NEA is a multidimensional extension of the reflection principle, an approach often used for estimating spectra in photodissociative systems. With popularization molecular mechanics, ensembles of geometries started to be also used to estimate the spectra through incoherent sums. Thus, different from the reflection principle, which is usually done via direct integration of analytical functions, the NEA is a numerical approach. In 2012, a formal account of NEA showed that it corresponded to an approximation to the time-dependent spectrum simulation approach, employing a Monte Carlo integration of the wavepacket overlap time evolution. NEA for absorption spectrum Consider an ensemble of molecules absorbing radiation in the UV/vis. Initially, all molecules are in the ground electronic state Because of the molecular zero-point energy and temperature, the molecular geometry has a distribution around the equilibrium geometry. From a classical point of view, supposing that the photon absorption is an instantaneous process, each time a molecule is excited, it does so from a different geometry. As a consequence, the transition energy has not always the same value, but is a function of the nuclear coordinates. The NEA captures this effect by creating an ensemble of geometries reflecting the zero-point energy, the temperature, or both. In the NEA, the absorption spectrum (or absorption cross section) σ(E) at excitation energy E is calculated as where e and m are the electron charge and mass, c is the speed of light, ε0 the vacuum