Can Atheism ground objective morality?

[The Tanager]

In my exploration and discussions, I have not seen an atheistic worldview be able to logically account for morality being objective. The closest I've seen is an atheistic platonic kind of moral system, but while that seems to be able to account for the existence of 'good' and 'evil', it still doesn't seem to account for why humans would be obligated to choose Good over Evil. I'd love to hear cases from those who think atheism can account for objective morality to make sure I haven't missed (or misunderstood) thoughts in my exploration.

[bluegreenearth]

In my exploration and discussions, I have not seen an atheistic worldview be able to logically account for morality being objective. The closest I've seen is an atheistic platonic kind of moral system, but while that seems to be able to account for the existence of 'good' and 'evil', it still doesn't seem to account for why humans would be obligated to choose Good over Evil. I'd love to hear cases from those who think atheism can account for objective morality to make sure I haven't missed (or misunderstood) thoughts in my exploration.

Are you defining "objective" to mean something that exists in reality such that its properties are not contingent on a mind?
Are you defining "morality" to mean a set of behavioral standards that consistently distinguishes conduct valued as "good" from conduct valued as "evil?"