perustila

Perustila is a Finnish term meaning "base state" or "ground state." In science, it refers to the lowest energy configuration of a system and serves as the reference point for describing excited states. The concept is used in physics, chemistry and related fields; in English, it is typically translated as "ground state" or "base state."

In quantum mechanics, the perustila is the stationary lowest-energy eigenstate of the system's Hamiltonian. It is usually time-invariant for an isolated system and minimizes the total energy. The energy difference to the first excited state, the energy gap, governs how the system responds to perturbations or thermal excitation. In chemistry, the ground electronic state of a molecule is the electronic configuration of minimum energy; electronic or vibrational excitations produce higher-energy states and give rise to spectra and photochemical processes. In many-body systems, the ground state can be degenerate, meaning multiple configurations share the same minimum energy.

The term is sometimes used more loosely to describe a default or baseline configuration of a system, model or experiment—an initial state from which changes are measured.

See also: ground state; initial state; default state.