We predict that the difference in quantum confinement energies of G-like and X-like conduction states in a covalent quantum dot will cause the direct-to-indirect transition to occur at substantially lower pressure than in the bulk material. Furthermore, the first-order transition in the bulk is predicted to become, for certain dot sizes, a second-order transition. Measurements of the "anticrossing gap" could thus be used to obtain unique information on the G-X-L intervalley coupling, predicted here to be surprisingly large (50 -100 meV). [S0031-9007 (98)06301-7] PACS numbers: 71.24. + q, 73.20.DxReduced dimensions usually cause pressure-induced structural phase transitions to occur at elevated pressures relative to the bulk solid. This is the case for the AlAs layers in AlAs͞GaAs superlattices [1], for the transition to b-Sn structure in Si nanocrystals [2], and for the wurzite-to-rocksalt structure in CdSe dots [3]. Here, we show that reduced dimensionality causes another type of pressure-induced transition-the electronic direct-toindirect transition-to occur at reduced pressures relative to the bulk.Pressure-induced direct ͑G 1c ͒ to indirect ͑X 1c ͒ transitions occur in bulk zinc blende semiconductors [4,5] because under pressure, the G 1c energy goes up while the X 1c energy goes down [Figs. 1(a) and 1(b)]. This reflects the fundamentally different charge distribution in these two states [6]: the antibonding G 1c state has a node along the cation-anion bond, so it is destabilized (moves up in energy) as this bond is shortened, while the X 1c state has most of its amplitude in the interstitial volume, where no atoms exist. As a result of the different signs of the G 1c and X 1c deformation potentials, in materials where at zero pressure the energy of the X 1c state is not too far above the G 1c state (GaAs 4 , InP 5 , but not InAs or CdSe), a pressureinduced first-order G 1c ! X 1c level crossing [7] occurs before the material is structurally phase transformed.Reduced dimensionality can alter the energetic separation between the G 1c -like and X 1c -like states even without pressure [ Figs. 1(b) and 1(c)]. This results from the fact that quantum confinement raises the energy of G 1c (with lighter mass) faster than the energy of X 1c (with heavier mass) [ Figs. 1(b) and 1(c)]. Thus, if the energy of the X 1c state is not too far above the G 1c state in bulk, reduced size alone can cause a direct-to-indirect transition to occur at zero pressure. Detailed calculations [8] without pressure effect predicted this to occur in GaAs films, wires, and dots as size diminishes. Because of the larger (ϳ0.95 eV, measured [9]) G 1c 2 X 1c separation in bulk InP relative to in bulk GaAs (0.55 eV [10]), no direct-toindirect transition was predicted to occur in free-standing InP dots at zero pressure [11]. Since, however, quantum confinement in InP dots could reduce the G 1c 2 X 1c energy separation relative to the bulk, it might take less pressure to transform the dot than to transform the bulk into an indirect band gap [ Figs...