First-principles　calculations　based　on　DFT-GGA　method　have　been　performed　on　rubidium　adsorption　on　Si(001)(2　x　1)　surface.　The　atomic　and　electronic　structures　of　Si(001)(2　x　1)-Rb　have　been　calculated　and　compared　with　those　of　Cs　adsorption　[H.Y.　Xiao,　X.T.　Zu,　Y.F.　Zhang,　L.　Yang,　J.　Chem.　Phys.　122　(2005)　174704].　It　turns　out　that　the　saturation　coverage　of　Rb　is　one　monolayer　rather　than　half　a　monolayer,　similar　to　that　of　Cs　adsorption.　Comparison　of　Rb　on　Si(001)(2　x　1)　with　Cs　adsorption　showed　that　at　saturation　coverage　larger　alkali　metal　(AM)　atom　leads　to　stronger　AM-AM　interaction　and　weaker　AM-Si　interaction.　However,　for　low　coverage　of　0.25　and　0.5　ML　the　Rb-Si　interaction　is　surprisingly　weaker　than　Cs-Si　interaction.　Further　detailed　analysis　suggested　that　this　is　a　consequence　of　depolarization　effect　with　decreasing　AM　size　below　I　ML　coverage.　For　the　saturation　coverage,　the　dispersion　curves　show　that　the　surface　is　of　semi-conducting　character.　This　result　does　not　support　the　direct　and　inverse　angle-resolved　photoemission　investigation　where　a　metallization　is　observed　at　saturation　coverage.　(c)　2005　Elsevier　B.V.　All　rights　reserved.