Verbs with Akkusativ Object: (1.) optional, (2.) obligatory. What's the name for these two categories?

Question

Rationale: I am making flashcards and I need to give myself tips to differentiate between similar words. Sometimes (actually in German very often) there are couples of transitive verbs that share the (almost) same meaning with the following grammatical difference:

(1.) in one the Akkusativ Objekt is compulsory;

(2.) in the other, the Akkusativ Objekt is an extra-information.

A basic example is kaufen and einkaufen. This is not the most illustrating instance as the meanings ARE different. Yet, I want to focus on the Grammar (if you are not happy with kaufen/einkaufen, think of begießen/gießen).

"kaufen" needs always an Akkusativ Objekt. The sentence "Ich kaufe" by itself is incomplete. Yet, one can say "Ich kaufe ein". This one can gramatically survive.

To reiterate my question: is there a proper name for the two categories of transitive verbs like kaufen and einkaufen?

Answer 1

No, I don't think so, at least I've never come across a specific term. We just differentiate between essential (obligatorisch) and non-essential (fakultativ) complements (~ "objects") and put the non-essential complements in parantheses. Ie, an essential accusative complement might be denoted as K_akk and a non-essential accusative complement as (K_akk). (There are different ways to represent complements but the use of parantheses to indicate optionality is a common feature.)

So, for instance, we could describe the valence pattern of kaufen very broadly as K_sub, K_akk, (K_dat), (K_prp), (K_adv), meaning that there is an essential subject complement (Ich kaufe eine Tasse.), an essential accusative complement to denote the purchased item (Ich kaufe eine Tasse.) [precisely speaking, it's not essential in all contexts: Wir kaufen nicht bei unhöflichen Verkäufern; but that's a side issue], a non-essential dative complement to denote the ultimate recipient of the purchased item (Ich kaufe meiner Mutter eine Tasse), a non-essential prepositive complement (eg with von to denote the actant from whom the item is bought: Ich kaufe eine Tasse von meinem Freund.) and a non-essential adverbial complement (eg with a für prespositional phrase to indicate the price of the item: Ich kaufe eine Tasse für zwei Euro). Meanwhile, the plan for einkaufen would look something like K_sub, (K_akk), (K_prp), (K_adv). That way, you can tell right away where you need an accusative and where not.