Below is the list of tables from the database along with their columns and datatype. Need to generate an accurate SQL query for the question.
Only use the columns present in the respective table. Only use the columns which are required to generate the SQL query. Only use table joins if required and not otherwise.

### Tables ###
TABLE: CREATE TABLE table_name_81 (population_density__per_km²_ VARCHAR, name VARCHAR, capital VARCHAR)

### Question ###
What is Population density (per km²), when Name is Antigua and Barbuda, and when Capital is St. John's?

### Accurate SQL ###

SELECT population_density__per_km²_ FROM table_name_81 WHERE name = "antigua and barbuda" AND capital = "st. john's"