SAN DIEGO, Calif.--After its initial public offering earlier this month, CombiChem announced the achievement of milestones in two of its collaborations. The computational chemistry company here offered 2.25 million shares at $8 per share in the offering managed by BancAmerica Robertson Stephens, Donaldson, Lufkin & Jenrette, and Salomon Smith Barney. CombiChem offered underwriters an option to buy an additional 337,500 shares of common stock to cover over-allotments.