Two novel numerically efficient approaches are proposed for maximum capacity designs in multiple-input multiple-output (MIMO) systems under the practical per-antenna power constraint. One is an explicit solution (ES) when the rank of the channel matrix is equal to the number of transmit antennas. And the other is an efficient iterative approach (EIA). The ES is optimum and the EIA is nearly optimum in the maximum capacity sense. The key advantage of the EIA is that the number of data streams (i.e., the rank of the transmit covariant matrix) can be pre-specified and therefore tradeoffs between multiplexing and diversity gains can be facilitated. Numerical results show that the capacity and MSE performances with the more practical per-antenna power constraint are very similar to those with the less practical total power constraint.