  The STEP (Stability, Transport, Equilibrium, and Pedestal)
integrated-modeling tool has been developed in OMFIT to predict stable, tokamak
equilibria self-consistently with core-transport and pedestal calculations.
STEP couples theory-based codes to integrate a variety of physics, including
MHD stability, transport, equilibrium, pedestal formation, and current-drive,
heating, and fueling. The input/output of each code is interfaced with a
centralized ITER-IMAS data structure, allowing codes to be run in any order and
enabling open-loop, feedback, and optimization workflows. This paradigm
simplifies the integration of new codes, making STEP highly extensible. STEP
has been verified against a published benchmark of six different integrated
models. Core-pedestal calculations with STEP have been successfully validated
against individual DIII-D H-mode discharges and across more than 500 discharges
of the $H_{98,y2}$ database, with a mean error in confinement time from
experiment less than 19%. STEP has also reproduced results in less conventional
DIII-D scenarios, including negative-central-shear and negative-triangularity
plasmas. Predictive STEP modeling has been used to assess performance in
several tokamak reactors. Simulations of a high-field, large-aspect-ratio
reactor show significantly lower fusion power than predicted by a
zero-dimensional study, demonstrating the limitations of scaling-law
extrapolations. STEP predictions have found promising EXCITE scenarios,
including a high-pressure, 80%-bootstrap-fraction plasma. ITER modeling with
STEP has shown that pellet fueling enhances fusion gain in both the baseline
and advanced-inductive scenarios. Finally, STEP predictions for the SPARC
baseline scenario are in good agreement with published results from the physics
basis.
