ABSTRACT:The ultraviolet (UV)-Visible absorption spectrum of trans-stilbene (tS) is computed at different temperatures by couplin g molecular dynamic s (MD ) simulations with the classical MM 3 force fiel d to ZINDO/S-CI S calculation s of vertical excitation energies and transition dipol e moments. The selection of a large number of structures along the MDtrajectories enables a consistent treatment of temperature effects in the vacuum , whereas the ZINDO/S-CI S calculation s permit a reliable treatment of electron correlation and relaxation, taking account of multistat e interactions in the fina l state. Thermal motions are found to alter very differentl ythe width and shape of bands. Structural alterations such as the stretching and the torsion of the vinyl single and double bonds very stronglyinfluenc e the appearance of the fi rst valence state, pertaining to the highest occupied and lowest unoccupied molecular orbital (HOMO-LUMO ) transition. At temperatures less than 400 K, these are found to yield a merelyGaussian and very pronounce d thermal broadenin g of the related band (A), up to nearly 30 nm, together with a minor blue shift of its maximum λ max . In contrast, a red shift by several nanometers occurs due to thermal motions for the remainin g three valence bands. As can be expected, the broadenin g intensifie s at higher temperatures, and for the A-band , becomes markedly asymmetric when T exceeds 400 K. The combinatio n of MD(MM3 ) and ZINDO/S-CIS computation s enables also consistent calculation s of hot bands, which are forbidden by symmetryat 0 K.