Scope ===== Current capabilities -------------------- NMRDfromMD is currently designed for the prediction and analysis of :math:`^{1} \text{H}` nuclear magnetic relaxation dispersion (NMRD) profiles from molecular dynamics trajectories. The current implementation focuses on relaxation mechanisms dominated by magnetic dipole-dipole interactions, which can be directly described through the time-dependent fluctuations of internuclear vectors. In particular, :math:`^{1} \text{H}` relaxation can be calculated from the rotational autocorrelation functions of relevant interatomic vectors extracted from MD trajectories, followed by the calculation of the corresponding spectral densities. The current framework is applicable to both isotropic and anisotropic molecular systems. For isotropic systems, such as bulk liquids or freely tumbling molecules, the relaxation analysis relies on the assumption of isotropic rotational diffusion. For anisotropic systems, such as confined liquids or molecules interacting with surfaces, the full orientation-dependent dynamics of internuclear vectors are retained, allowing the effect of restricted or heterogeneous molecular motions on the relaxation dispersion profile to be investigated. This enables the study of systems where deviations from simple isotropic rotational diffusion play a key role in determining the observed NMR relaxation behaviour. Future extensions ----------------- Currently, NMRDfromMD is primarily focused on the prediction and analysis of :math:`^{1} \text{H}` nuclear magnetic relaxation dispersion (NMRD). This focus is motivated by the relatively simple and well-established relationship between proton relaxation rates and molecular motion, which is often dominated by dipole-dipole interactions that can be directly described through the autocorrelation functions of internuclear vectors extracted from molecular dynamics trajectories. Extending the framework to other nuclei, such as :math:`^{13} \text{C}`, :math:`^{15} \text{N}`, and :math:`^{19} \text{F}`, is considerably more challenging because different relaxation mechanisms can become dominant depending on the chemical environment. For example, :math:`^{13} \text{C}` relaxation may be governed by heteronuclear (:math:`^{13} \text{C}`-:math:`^{1} \text{H}`) dipolar interactions for protonated carbons, whereas carbonyl or quaternary carbons require the treatment of additional contributions such as chemical shift anisotropy (CSA) and tensorial fluctuations. Similarly, relaxation analysis for nuclei such as :math:`^{15} \text{N}` often requires the combined description of dipolar and CSA mechanisms. These extensions therefore require additional theoretical developments, including the calculation of anisotropic interaction tensors and their time-dependent fluctuations from MD trajectories. Although not currently implemented, expanding NMRDfromMD toward multi-nuclear relaxation analysis represents a natural future direction for the package. Out-of-scope relaxation mechanisms ---------------------------------- While NMRDfromMD is designed to describe relaxation processes that can be directly related to molecular motions extracted from classical MD trajectories, some relaxation mechanisms require additional physical information beyond the current framework. These mechanisms involve interactions that are not fully captured by standard nuclear dipole-dipole relaxation models, such as fluctuations of electric field gradients or electron spin dynamics. The following sections describe relaxation processes that are currently outside the scope of NMRDfromMD and would require significant theoretical and methodological extensions. Quadrupolar relaxation ~~~~~~~~~~~~~~~~~~~~~~ NMRDfromMD is currently not intended for systems where quadrupolar interactions represent the dominant relaxation mechanism. Quadrupolar nuclei, such as :math:`^{2} \text{H}`, :math:`^{14} \text{N}`, or other nuclei with spin quantum numbers greater than :math:`1/2`, require a different theoretical treatment because relaxation arises from fluctuations of the electric field gradient (EFG) tensor rather than only from magnetic dipole-dipole interactions or chemical shift anisotropy. Predicting quadrupolar relaxation from MD trajectories would therefore require the calculation of the time-dependent EFG tensor at the nucleus, either from suitable force-field descriptions or from quantum-mechanical calculations, followed by the evaluation of the corresponding spectral densities. These developments involve additional methodological and computational challenges and are therefore outside the current scope of NMRDfromMD. Paramagnetic relaxation ~~~~~~~~~~~~~~~~~~~~~~~ NMRDfromMD is also currently not designed for systems where paramagnetic centers provide a significant contribution to nuclear relaxation. In the presence of unpaired electron spins, such as in systems containing transition-metal ions or other paramagnetic species, relaxation can be dominated by electron-nuclear dipolar interactions and contact interactions. These mechanisms depend on the dynamics of the electron spin, the electron-nuclear distance distribution, and the electron spin relaxation properties, which are not directly accessible from standard classical molecular dynamics trajectories. A quantitative description of paramagnetic relaxation would therefore require additional information, such as electron spin relaxation times, hyperfine coupling parameters, or magnetic susceptibility tensors, often obtained from quantum-mechanical calculations or specialized spin dynamics approaches. Such extensions represent a major increase in theoretical complexity and are therefore outside the current scope of NMRDfromMD.