# Modality 1: Inverse Band Structure

*Modality 1 applies to cases where we have a single material system, but an astronomical number of configurations, and where the target properties can be calculated on the fly.*

The approach is also called Inverse Band Structure (IBS). The IBS approach began a dozen years ago within the Solid-State Theory group at the National Renewable Energy Laboratory (NREL), under support from the U.S. Department of Energy's Office of Basic Energy Sciences.

Imagine that you have a constrained material system such as an alloy, impurity, or a nanostructure. In each of these cases, we might have a huge number of spatial configurations, (e.g., different realizations of A and B atoms on a lattice with N points).

In principle, each configuration (alloy, nanostructure, or impurity system) would have a different value of the physical property of interest (e.g., bandgap, Curie temperature, oscillator strength for absorption, defect level depth). Clearly, one cannot hope to calculate all configurations, but must calculate a few and infer the rest. The computational strategy most appropriate is the use of *sampling methods* (such as Simulated Annealing or Genetic Algorithms) in conjunction with the relevant quantum mechanics. Thus, we calculate a few thousand configurations and seek to find the one with the target property out of ˜10^{14} configurations.

In Modality 1, we assume that a configuration can be realized synthetically even if it is not a ground state, e.g., superlattice sequences or different core-shell nanostructures. Historically, NREL's Solid-State Theory group developed the IBS methodology for alloys and impurities prior to the Energy Frontier Research Center.

Also check Modality 2 and Modality 3. The three modalities are a framework for the evolving application of inverse design.