In 2014 the Department of Astronomy at Stockholm University has started a theoretical/computational solar physics group. Currently it consists of a professor, five researchers, a postdocs and four PhD students. The aim of the group is to understand the physics of the atmosphere of the Sun. Magnetism underlies much of the activity and dynamics of the solar atmosphere. Observations with observatories such as SDO, IRIS and the Swedish Solar Telescope show an intricate structure and dynamics form the smalls observable scale to the size of the entire Sun. These phenomena are ultimately driven by the nuclear fusion in the solar centre and the enigmatic solar dynamo. The physics of the atmosphere is complex, encompassing the interaction of (polarised) radiation and matter, magneto-hydrodynamics and plasma physics. Understanding the observations in terms of the physics thus requires intricate theoretic modelling. The project "Physics of the solar atmosphere" aims to contribute to our understanding of the solar atmosphere through (1) radiation-MHD modelling of the atmosphere, (2) precise modelling of the spectral lines from these MHD models that can be directly compared to observations and (3) inversion of observations. Inversion is an iterative technique where an atmospheric structure is derived that is consistent the observed spectral line profiles. The group uses a variety of codes for which the PI and Co-Is are co-developers. All codes have been run before on both NAISS and non-NAISS supercomputers. We are performing regular production runs on NAISS resources and have published 5 refereed papers using NAISS resources during April 2024 -April 2025. We expect to submit ~5 papers to refereed journals during 2025-2026. We ask for a total of 500,000 h/month on Dardel.