An approximation technique used to calculate quantities in quantum mechanics. This technique is called the semiclassical approximation because the wave function is written as an asymptotic series with ascending powers of the Planck constant h, with the first term being purely classical. The semiclassical approximation is also known as the Wentzel–Kramers–Brillouin (WKB) approximation, named after its inventors Gregor Wentzel (1898–1978), Hendrik Anton Kramers (1894–1952), and Léon Brillouin (1889–1969), who invented it independently in 1926. The semiclassical approximation is particularly successful for calculations involving the tunnel effect, such as field emission and radioactive decay producing alpha particles.