Single keywords words cannot be promoted effectively.
For example, it is not likely that someone looking for “free accounting software download” is going to type just “software” into the search box.
Avoid the most popular keyword phrases because you’ll be competing with millions of other pages for a search engine’s attention. It is unrealistic to think that a new website could rank number one on a popular phrase like “mp3”. More established companies who have been on the Internet for several years will have the big advantage of a high link popularity.
People who whose multiple word keywords are in the compare or buying phase. They are more likely to purchase goods or services than those using fewer words (source: Oneupweb Research).