As I’ve mentioned in the comments – you shouldn’t need to modify / touch WordPress globals directly. It seems you are getting the errors because you are trying to load WordPress within Smart – this is a bit of a hacky approach.
I’ve not used Smarty at all, but WordPress has its own way of handling templates and trying to load WordPress within another framework is not going to be without its difficulties. Instead, you might consider this plug-in:
http://wordpress.org/extend/plugins/smarty-for-wordpress/ which allows you to use your smarty template with WordPress. To quote the plug-in’s readme:
Smarty for WordPress is the first plugin which incorporates a complete distribution of the Smarty template engine as a WordPress plug-in. You embed your Smarty template file by using a WordPress short code with the Smarty template file name and any variable which you want to be passed to your Smarty template file.