models.physics.particles.fields gives particle-carried fields an initial value - today Psi - as a number, an expression in physical x y z, normalized xn yn zn, pid, t and per-particle uniform()/normal() draws, or regions (half_space, slab, box, ball, cylinder, with optional smoothed edges) painted over a background. Values are applied before the first scatter and summarized in particle_initial_fields.csv. The expression language is shared with ic_gen, which gains and, or, not, and chained comparisons.