<tt>PICASO 4.0</tt> : Clouds and Photochemistry in Climate Models of Brown Dwarfs and Exoplanets

Abstract We present a major update to the open-source atmospheric modeling package PICASO , designed for simulating the thermal structure and spectra of hydrogen-rich atmospheres of brown dwarfs and exoplanets. This release, PICASO 4.0 , expands upon the existing radiative-convective equilibrium model framework by incorporating several new capabilities. Key additions include the integration of Virga for self-consistent cloud modeling, new flexible treatments for rainout and cold trapping of volatile species, and support for photochemistry. We also introduce a parameterized energy injection scheme to simulate additional external or internal heating processes. These features are motivated by lessons from recent JWST observations that reveal the prevalence of nonequilibrium chemistry and clouds. We benchmark the new functionalities against previously published results in the literature, including the Sonora Diamondback grid, energy injected atmospheres, patchy cloud models, and other photochemical models of WASP-39b. PICASO continues to be actively developed as an open-source package aimed at enabling reproducible, community-driven atmospheric modeling of all substellar objects.