In this study we employed the B3LYP/6-311 +G(d,p) method combined with the CIS/6-311+ +G(d.p) calculation to investigate the effects of the type and the number of alkali metal atoms (Li, Na, K) on the geometric, electronic, and optical properties of alkali metals substituted into adamantanes. Substituting alkali metal (Li, Na, K) atoms caused significant changes in the electronic and optical properties of adamantane. The Ad-1Li, Ad 1Na, and Ad-1K structures showed a dramatically decreased energy gap and ionization potential, while adding more alkali metal atoms slightly decreased these properties. Substituting more alkali metals led to a shift in the maximum absorption wavelength from the visible to the infrared region, depending on the type of alkali metal atom substituted. The magnitude of shift occurred in the following order: Li < Na < K. These characteristics suggest the possibility of tunable electronic structures of this material for optoelectronic device applications. (C) 2018 The Chemical Industry and Engineering Society of China, and Chemical Industry Press. All rights reserved.